Aura/docs/design/INDEX.md

# aura design ledger — INDEX

The ledger records the load-bearing design contracts and their rationale. Each
contract states what it **guarantees**, what it **forbids**, and **why**. A
change that breaks a contract is a design decision (amend the contract here with
its new rationale), never a silent refactor.

Provenance: contracts C1–C18 were settled in the initial rough-sketch design
interview (2026-06-03), walking the design tree root-to-leaf (C16–C18 and the
C10 refinement to a broker-independent position table came in follow-up turns;
C10 was later reframed in cycle 0007 — the intent/exposure stream is the primary
output, the position table a derived layer — see C10). C19–C22 were added as the
construction / World / playground layer; C23 and the C9/C19 compilation
refinements were settled 2026-06-05 for the **Construction-layer** milestone (the
blueprint→compilat reading of composites and graph optimisation).
The `CLAUDE.md` **Domain invariants** section is the compressed, always-loaded
summary of the
subset that agents must never violate; this file is the fuller form with
rationale.

Vocabulary: a *contract* is one ledger entry. A *cycle* is one pipeline round; a
*milestone* is a tracker container spanning many cycles (the first milestone is
the **walking skeleton**: ingest → one signal → exposure/intent → sim-optimal
broker → synthetic pip-equity signal-quality metric).

---

## Foundation — what aura is

aura is a framework **and** a playground for traders. A human and (primarily)
LLMs author trading **nodes** directly in Rust; the engine backtests them
deterministically and massively in parallel, composes them fractally, validates
them (sweep / Monte-Carlo / walk-forward), and freezes a validated strategy into
a standalone bot with a broker connection.

The predecessor RustAst (`myc`) tried this as a custom DSL and failed: too slow,
too buggy, and LLMs author far better in Rust than in an unfamiliar DSL. aura
inverts it — engine in Rust, strategies in Rust — but keeps RustAst's *concepts*
(synchronous reactive streams, bounded-lookback series, run-counting, SoA).
RustAst is a conceptual reference, not a dependency. The one reused component is
`data-server` (the first data source).

What sets aura apart is **not** the single backtest — every quant system does
that — but the **World**: the meta-level where harnesses are dynamically
constructed and *families* of them orchestrated and explored (walk-forward,
sweeps, Monte-Carlo, comparison). The deterministic single-harness engine is the
*substrate*; the World is the *product* (C20–C22).

---

## External components

Two sibling projects live outside this repo and are named throughout the
contracts. Their concrete location is recorded here so the repo is the single
source of truth — not session memory.

- **`data-server`** — `~/dev/libs/data-server`, Gitea `Brummel/data-server`
  (`http://192.168.178.103:3000/Brummel/data-server.git`). aura's **first data
  source** and the one reused component (Foundation; C3, C11, C12). A standalone
  leaf crate (deps: `chrono`, `regex`, `zip`) that loads Pepperstone **M1/tick**
  binary files and shares them lock-free as `Arc<[T]>` chunks
  (`CHUNK_SIZE = 1024`) via `SymbolChunkIter::next_chunk` (an inherent method
  driven by `while let`, not the `Iterator` trait); `stream_*_windowed(from_ms,
  to_ms)` provides C12's data-window with inclusive Unix-ms bounds. Records are
  **AoS** (`M1Parsed`/`TickParsed`, `time_ms: i64` Unix-ms). The ingestion
  boundary (C3) therefore **transposes** these AoS records into aura's SoA
  columns (C7) and normalizes `time_ms` → canonical epoch-ns at that one
  boundary. Pulled in as a cargo git dependency by the **`aura-ingest`** crate
  (cycle 0011) — the ingestion edge and the workspace's **external-dependency
  firewall**: `aura-ingest` is the *only* crate that links `data-server` (hence
  its transitive `chrono`/`regex`/`zip`), so the engine crates (`aura-core` /
  `aura-std` / `aura-engine` / `aura-cli`) and the frozen deploy artifact (C13)
  stay external-dependency-free. Consequence: `cargo build/test --workspace` now
  resolves from a populated cargo cache (one Gitea fetch) rather than fully
  offline.
- **RustAst (`myc`)** — `~/dev/RustAst`, Gitea `Brummel/RustAst`
  (`http://192.168.178.103:3000/Brummel/RustAst.git`). The predecessor DSL attempt
  (Foundation): **a conceptual reference, never a dependency** — its DSL authoring
  surface and `Value`/HM-inference machinery are exactly what aura rejects (C17).
  But its **`src/ast/rtl/` layer is a working reference implementation of the very
  streaming substrate aura rebuilds**, and is worth reading before authoring
  `aura-core` — these are not just concepts, they exist as code:
  - `rtl/series/data.rs` — `RingBuffer<T>` with financial-style indexing (index 0
    = newest), `total_count` (the **run-count** of C5) and `lookback_limit` (C8's
    pre-sized window); the `ScalarValue` marker trait ("flat scalars only, no
    String/Record" = C7's closed scalar set); `ScalarSeries<f64|i64|bool>` and the
    **SoA** `RecordSeries` for composites (C7's "OHLCV = a bundle of base columns").
  - `rtl/series/mod.rs` — `create_typed_series`, dispatching element type → storage
    backend (the type-specialized **factory** of C19).
  - `rtl/streams/mod.rs` — `Signal { cycle_id, value }` (C4's cycle clock) and the
    `Stream` / `Observer` / `ObservableStream` push traits (the reactive model of
    C4/C5).
  - `rtl/streams/register.rs` — the RTL **"register" / delay node** (the one
    explicit feedback path of C5/C9) plus a seeded, reproducible OHLC generator
    (C12's seed-as-input).

  aura reimplements these natively and **sharpens** them: types are monomorphized
  and edges type-erased to the four scalar kinds with direct dispatch (C7) instead
  of carrying boxed `Value`s, and input history is shared zero-copy as `Arc<[T]>`
  (C12) instead of a `VecDeque<Value>`. RustAst shows the *shape*; aura makes it
  fast and deterministic.

---

## Contracts

### C1 — Determinism and disjoint parallelism
**Guarantee.** A backtest is a deterministic, synchronous, non-concurrent event
loop that reaches a unique state after each input tick. Same input (incl. seed)
→ bit-identical run. Two backtests are fully disjoint and run concurrently
without locking.
**Forbids.** Concurrency *within* a single sim; any nondeterministic input that
is not captured as an explicit input (see C11, C12).
**Why.** Real money rides on backtest results; reproducibility and an audit
trail are non-negotiable. Speed comes from parallelism *across* sims, which
disjointness makes lock-free.

### C2 — Causality / no look-ahead
**Guarantee.** A node sees only the past. Input history is a read-only window
that ends at the current cursor; a resampler emits a bar only once it is
complete.
**Forbids.** Any node access to data with `timestamp > now`; emitting a partial
/ still-forming bar.
**Why.** Look-ahead is the cardinal backtester bug — a fast backtester that
leaks the future is worse than none. Making the future *physically absent* from
what a node receives beats merely discouraging it.

### C3 — One merge, at ingestion only
**Guarantee.** Heterogeneous timestamped sources are k-way-merged by timestamp
into one chronological cycle stream at the ingestion boundary; source-native
time units (e.g. data-server's Unix-`time_ms`) are normalized there to the
canonical epoch-ns `timestamp` of C7.
**Forbids.** Any merge / as-of join *inside* the graph.
**Why.** A single ordered timeline is the mechanism that makes heterogeneous-rate
sources (news daily-bias + M5 + ticks) causally combinable without leaking the
future. Keeping the merge at one boundary keeps the graph semantics simple.

### C4 — Cycle granularity
**Guarantee.** The clock is data-driven: one input record = one cycle, advanced
in global timestamp order, with a monotonic `cycle_id`. Ties (same timestamp,
multiple sources) break by source declaration order.
**Forbids.** A fixed time-grid clock; nondeterministic tie ordering.
**Why.** The market *is* an irregular event sequence; a grid is arbitrary and
either wastes empty cycles or clumps ticks. Backtest and live differ only in the
origin of records, not the cycle semantics. Tie determinism preserves C1.

### C5 — Freshness-gated recompute and sample-and-hold
**Guarantee.** The `cycle_id` advances everywhere (a cheap counter), but a node
re-evaluates only when ≥1 of its own inputs is fresh this cycle (detected by
run-count); otherwise it holds its last output. Stale inputs contribute their
last (held) value.
**Forbids.** Recomputing every node every cycle ("push all" is true for the
*clock*, not for *recompute"); treating a held value as missing.
**Why.** Total recompute does not scale to many sparse high-frequency sources;
freshness-gating is the performance discipline that keeps the synchronous model
fast.

### C6 — Firing policy A and B, per input group
**Guarantee.** A node declares, per input group, one of two firing policies:
**A** fire-on-any-fresh + hold (latest / as-of join — e.g. tick × held
daily-bias); **B** all-fresh barrier (synchronizing join — e.g. O/H/L/C from
four separate 15m sources: the candle is complete only when all four are fresh).
A single node may mix an A input and a B group.
**Forbids.** A single global firing mode; forcing per-node-only granularity.
**Why.** Both are genuinely needed; RustAst implemented only B. Per-input-group
granularity is required by the OHLC-plus-bias case where one node needs both.
**Realization (cycle 0004).** Firing is tagged per input — `Firing::{Any,
Barrier(u8)}` on `InputSpec` — and inputs sharing a `Barrier` id form a group; a
mode-A input is its own trivial group, so "per input group" and the per-input tag
coincide. The barrier's synchronization token is the cycle **timestamp**, not the
`cycle_id`: under C4 four same-timestamp sources are four distinct cycles, so
RustAst's `cycle_id`-equality barrier could never fire across them. A group fires
when every member's last push carries the current cycle's timestamp, guarded by
"≥1 member fresh this cycle" (so a group completed earlier does not re-fire). This
fires both the multi-source bar and the within-source diamond rejoin (every push
in a cycle carries that cycle's timestamp).

### C7 — Four scalar base types, streamed as SoA
**Guarantee.** Only `i64`, `f64`, `bool`, `timestamp` (newtype over i64,
epoch-ns UTC) are streamed, as columnar Structure-of-Arrays. Composite streams
(OHLCV) are bundles of base columns — this is the **node-output model** too: a
node emits a record of 1..K base columns (C8), each forwarded field-wise to a
consumer slot; the bundle is structural, never a fifth scalar type. Edges are
type-erased to these four kinds;
the type check is paid once at wiring/sim-start, then the topology is frozen per
sim → direct dispatch, no per-event allocation.
**Forbids.** Streaming non-scalars (String, Records, tables, calendars) — those
live as metadata beside the hot path; `dyn Any` payloads; per-event heap
allocation; topology mutation mid-sim.
**Why.** Maximal streaming performance (SIMD/cache) needs a tiny closed scalar
set and SoA. The open set is composites (schemas of columns), not scalar types.
Type-erasure at the edge is also forced by the cdylib boundary (C13).

### C8 — The node contract
**Guarantee.** A node has a **signature** — its `NodeSchema`: each input's scalar
type and firing group, its output record, **and the node's own tunable parameters —
typed, with ranges**, which aggregate into the blueprint's param-space the optimizer
sweeps (C12/C19/C20). The signature is **declared pre-build on the value-empty
recipe** (C19), so the whole interface is legible without building (cycle 0024).
The built node implements `lookbacks()` — the per-input buffer depth, the one
quantity that may depend on an injected param (e.g. an `Sma`'s window = its
`length`), read once at bootstrap to size the windows — and `eval(ctx) ->
Option<&[Scalar]>`. The
engine provides read-only, zero-copy windows into each input's SoA ring buffer
(`ctx.f64_in(x)[k]`, sized at wiring); a node may *additionally* keep its own
mutable series for derived/intermediate state. `None`/Void return = filter /
not-yet-warmed-up. A node is a **producer, a consumer, or both**: a
producer/transformer exposes **one output port**, whose payload is a **record of
1..K base-scalar columns** (a scalar is the degenerate K=1 record; an `eval`
returns a borrowed row, one value per column); a **pure consumer (sink)** — chart,
equity, logger — has **no** output. Sources are pure producers; sinks are pure
consumers.
**Forbids.** A node sizing/growing its input lookback at runtime; more than one
output **port** per node; a fifth scalar type or a heterogeneous output payload
(a record is a bundle of base columns, C7); copy-on-read of input history.
**Why.** Engine-provided windows mean LLM-authored code cannot mis-manage
lookback bookkeeping, and history passes through zero-copy. Fixed, pre-sized
buffers suit deterministic, pre-dimensioned sims (no realloc in the hot loop).
**Realization (cycle 0005).** `NodeSchema.output` is a `Vec<FieldSpec>` (named base
columns; length 1 = scalar). Binding is **field-wise only**: `Edge::from_field`
selects one producer column per edge; consuming a whole record is N edges (no
"bind whole record" mechanism). The K fields of one record are **co-fresh by
construction** (one `eval`, one timestamp), so C6 is untouched. `eval` returns
`Option<&[Scalar]>` — a borrowed row into a node-owned buffer — so the forward
path allocates nothing per cycle (C7).
**Realization (cycle 0006).** The pure-consumer (sink) half of this contract is
now realized at the substrate: **recording is a node role, not a type.** A
recording node reads its typed input windows + `ctx.now()` in `eval` and pushes
the record to a destination it holds as a field (a channel, a chart handle) — an
**out-of-graph side effect**. There is no `Sink` type, trait, or engine flag: a
node that only records returns `None` (pure consumer), and a node may record
**and** return an output the engine forwards in the same `eval` (the "both"
case). **Encoding & return contract.** A pure consumer declares `output: vec![]`
— the empty record *is* the sink declaration; there is no separate type, trait,
or marker. Its `eval` returns `None` or a zero-width `Some(&[])`, and the run
loop debug-asserts the returned row's width equals the declared output width
(`row.len() == schema.output.len()`). Field-wise wiring resolves
`Edge::from_field` against the producer's `output` at bootstrap, so no edge can
bind a field of a zero-output node — it fails with `BadIndex` — making a sink
structurally unwireable as an in-graph producer; its only output is the
out-of-graph side effect. In-graph routing stays engine-owned data (the edge table); the escape out
of the graph is the node's own side effect — and that boundary is the
determinism / graph-as-data boundary (C1/C7).

**Refinement (Construction-layer milestone — render labels, 2026-06-05).** A node
additionally exposes `label() -> String`, a **single-line, non-load-bearing**
render symbol: a default trait method the run loop never calls (wiring is by
index, C23). Overrides carry the node's identifying params (`SMA(2)` vs `SMA(4)`)
so a graph render (C9 graph-as-data, #13) disambiguates identical node types and
surfaces a mis-wiring. Like `FieldSpec.name`, it is an informative debug symbol,
not part of the C8 dataflow contract — adding it changes no run behaviour.

**Realization (cycle 0015 — param declaration).** The tunable-parameter half of
this contract is now realized: a node declares its knobs in `schema()` as
`params: Vec<ParamSpec>` (`ParamSpec { name: String, kind: ScalarKind }`), and
`Composite::param_space()` (cycle 0024; was `Blueprint::param_space()`) aggregates
them into one flat, path-qualified list — a
read-only projection of the graph-as-data (C9), mirroring the inline order
(C19/C23) so a param's slot matches the later flat-node order. Two refinements to
the guarantee's "typed, with ranges": (1) the declaration carries `name` + `kind`
only — the **search-range is the run's, not the node's** (which subset / grid a
sweep covers is an experiment axis, #32/C20; the node declares the knob's existence
and type, never its search interval). (2) Identity is **positional** (the slot,
C23 "by index, not name"); the path-qualified `name` is a non-load-bearing debug
symbol (like `FieldSpec.name`) — same-type siblings in one composite share a name,
uniqueness is at the slot. A vector knob (`LinComb.weights`) expands to `N` flat
`weights[i]` entries, `N` topology-fixed (C19). Permitted kinds are `i64`/`f64`/
`bool` (a `timestamp` knob is a structural axis, C20, never a numeric sweep param).
Binding a value to a slot landed in cycle 0016 (#31, see C19/C23); enumerating a
sweep (#32) is the deferred next layer. The 0016 binding also moved the param
declaration's *authoring* home into the value-empty recipe, whose `params()` is read
pre-build. In 0016–0023 the built node's `schema().params` still reported the same
slots, kept in lockstep by a per-node test (a duplication filed as debt, #36); **cycle
0024 dissolved this** — the signature is declared *once* on the recipe and the built
node no longer carries `schema()` (#36 closed). See the C8 cycle-0024 realization.

**Realization (cycle 0024 — the signature lives in the blueprint, #43/#36).** The
node's whole declared interface — its `NodeSchema` (input scalar types + firing,
output record, params) — now lives **once**, on the value-empty recipe
`PrimitiveBuilder` (ex-`LeafFactory`), read pre-build by `param_space()`, the
compile-time structural validation, and the render. This closes two debts: **#43**
(a value-empty recipe used to declare *no* input/output interface pre-build — only
params) and **#36** (params declared twice, recipe vs built node, kept in lockstep
by a per-node test — those 8 tests are deleted, the duplication structurally gone).
The split that makes this work: a node's signature is **fully static** per blueprint
(input kinds/firing, output fields, params — verified across the roster; a variable-
arity node like `LinComb` takes its arity as a *recipe argument*, not an injected
param), so it can be declared without building; the **one** param-dependent quantity,
an input's buffer **lookback** depth, is no longer in the signature but answered by
`Node::lookbacks() -> Vec<usize>`, read only by bootstrap to size the windows.
`Node::schema()` is therefore **removed** — the signature is pre-build data, not a
built-node method. `BlueprintNode::signature()` answers uniformly for both arms (a
primitive returns its recipe's schema; a composite *derives* it from the interior:
role kinds in, re-exported field kinds out, aggregated params), so "every node has a
signature in the blueprint" holds for composites too.

**Realization (cycle 0027 — name input ports, refs #21/#51).** `PortSpec` now
carries a `name: String` (it drops `Copy`, like `ParamSpec`), so an input port is
named just as `FieldSpec.name` (output) and `ParamSpec.name` (param) already are.
The name is **non-load-bearing** (C23): wiring stays positional by slot, bootstrap
and the run loop never read it; it exists for tracing / graph rendering (#13). Leaf
primitives declare their slot names (`SimBroker`'s `exposure`/`price`, etc.);
`derive_signature` carries a composite's `Role.name` into the derived input port,
so the graph model (`model_to_json`) is homogeneously named across inputs, outputs,
and params and across both graph levels. This does **not** close #21 (a swapped
same-kind wiring is still only kind-checked) — it makes those slots
self-documenting; a name-consuming validation is its own future cycle.

### C9 — Fractal, acyclic composition
**Guarantee.** A composite is itself a `Node` that wires a sub-graph and exposes
one output; signal, combined signal, and (with execution) strategy are all the
same abstraction, nestable arbitrarily. The dataflow graph is a DAG; the only
feedback path is an explicit delay/state node (the RTL "register"). Wiring is
written in Rust (builder API); the built graph is introspectable runtime data.
**Forbids.** Implicit dataflow cycles (combinational loops); special-casing
"signal-of-signals" as separate mechanics.
**Why.** Self-application of one contract gives unlimited composition with no
adapter zoo. Acyclicity keeps the synchronous reactive model well-defined;
forcing feedback through a visible delay node keeps the per-cycle determinism
intact and the one legitimate feedback path explicit. Graph-as-data enables
visualization, freezing, and re-parameterization for sweeps.
**Refinement (Construction-layer milestone, 2026-06-05).** "A composite is itself
a `Node`" is an **authoring-level** identity: a composite declares the same
interface (typed inputs + ≤1 output, C8) and is wireable wherever a node is, but it
is **not** a runtime object. The bootstrap **compiles it away by inlining** its
sub-graph into the one flat instance (C19/C23): the composite *boundary* dissolves
into the raw index wiring the run loop already consumes, and names stay
**non-load-bearing** (informative debug symbols only, as `FieldSpec.name` already
is). So "nestable arbitrarily" and "graph-as-data" hold at the **blueprint
(source)** level; the running graph is the flat, index-wired **compilat**. The earlier reading — *a composite survives as a `Box<dyn Node>`
driving a nested sub-engine* — is **explicitly rejected**: it would keep the
interior opaque to the cross-graph optimiser (C23) and add a runtime sub-loop the
flat model does not need. Inlining is what makes the composite boundary free.
**Realization (cycle 0018 — composite multi-output record, #40).** "Exposes one
output" is one output **port** carrying a **record of 1..K re-exported fields**, not
one field: `Composite.output` is a `Vec<OutField { node, field, name }>` (was a
single `OutPort`). Each entry is a **named projection** of one interior
`(node, output-field)`; a consumer selects which re-exported field it reads via the
same `Edge::from_field` that already binds leaf record columns (C8 realization,
cycle 0005). This is the **same arity** C8 already grants a leaf (OHLCV = one port,
5 columns): a multi-line indicator (MACD = macd/signal/histogram) is **one** record
of K fields, **one** port, **one** row per `eval` — C8/C7/C4 untouched, a boundary
completion, not a contract change. A strategy composite is simply the K=1 case
(one exposure field, C10). The re-export **names** are **non-load-bearing** (C23):
they live at the blueprint boundary and in render (cycle 0022/#46 folds each onto
its producing node as a `name := …` binding; originally `[out:<name>]` markers, #13)
but are dropped at lowering — `ItemLowering::Composite.output` is `Vec<(usize,
usize)>`, raw index pairs only, so the compilat is name-free (verified: the
compiled-view render stayed bit-identical across this change).
**Realization (cycle 0019 — name the composite boundary, #41).** The same
named-projection shape now covers the other two boundary edge-kinds, so **all
three** are uniform: `input_roles` is a `Vec<Role { name, targets }>` (was a bare
`Vec<Vec<Target>>`) and a composite carries `params: Vec<ParamAlias { name, node,
slot }>`, alongside `output: Vec<OutField>`. Each is an ordered, positionally-indexed
**named projection** of interior handles. Param aliasing is a **pure naming overlay**,
not curation: `param_space()` relabels an aliased slot's surface name **in place** —
slot order, arity, and kinds are unchanged, so the sweep surface is identical and the
`param_space_mirrors_compiled_flat_node_param_order` anchor stays green. Like the
output names, role and param **names** are **non-load-bearing** (C23): they live at
the blueprint boundary and in render (`[in:<name>]` / `[param:<name>]` markers) but
are dropped at lowering — the compilat is wired by raw index and a dangling alias is
rejected at compile (`BadInteriorIndex`), not silently lowered. The full composite
boundary signature (named inputs, params, multi-outputs) is now legible without
changing the compilat.
**Refinement (fan-in distinguishability, 2026-06-08).** A fan-in node (>1 input)
is well-formed only if its colliding sources — sources with equal recursive
signatures (type initial + alias initials + recursive input signatures) — do not
hide an unnamed configuration axis: a collision is a `CompileError`
(`IndistinguishableFanIn`) when at least one colliding source carries an
unaliased param slot. Genuinely-interchangeable sources (equal signatures, no
param) stay legal. Construction-phase only; the compilat stays name-free (C23).
The graph view renders each fan-in input as the shortest sibling-unique prefix
of its source signature.

### C10 — Strategy output is an intent/exposure stream; position management is a decoupled derived layer; brokers are downstream nodes
**Guarantee.** A strategy's primary, backtestable output is **not** an equity
curve, **nor a position-event table**, but an **intent / exposure stream**: the
DAG expresses exactly one *state* at time t (C8 — a node emits at most one record
per `eval`), so a strategy emits one **signed, bounded exposure** `f64 ∈
[-1, +1]` per cycle (per instrument; the portfolio is multi-instrument). Exposure
is the desired fractional position; **position sizing and risk live in the
decision/sizing node** that shapes a raw signal score into exposure. The chain is
`signals (scores) → decision/sizing node → exposure stream`.

**Signal quality** is evaluated by the **(a) sim-optimal broker** — a downstream
consumer node (C8/C9) that consumes the **exposure stream** plus the relevant
price stream and integrates `exposure(t-1) · (price(t) − price(t-1))` into a
**synthetic equity stream in pips**: deterministic, frictionless, perfect-fill
(no currency, no real-broker constraints). The exposure held *into* a cycle
(decided at t-1) earns that cycle's return, so the integration is causal (C2 — no
look-ahead). Pip PnL uses per-instrument pip metadata (reference data beside the
hot path, C7/C15). This pip-equity curve measures the **signal's quality**, not
an execution-modelled P&L; it is the primary research loop — backtest one signal,
combine it with another, backtest the combination.

**Position management** — turning the exposure stream into a broker-independent,
time-ordered **table of position events** (pure scalar columns, C7: `event_ts:
timestamp`, `action: i64` buy / sell / close, `position_id: i64`,
`instrument_id: i64`, `volume: f64` unsigned — direction is the `action`; a
position's open time is its opening event's `event_ts`, no separate `open_ts`; a
`close` references a `position_id` and may be partial via its own `volume`) — is a
**decoupled, derivable, downstream** layer. The events are the **first difference
of the exposure state**, materialized as a *computed table* (where multiple
events may share one `event_ts` — e.g. a reversal's close + open), **never as a
per-`eval` node output** (which C8 caps at one record per cycle). This layer feeds
**(b) realistic broker nodes** (Pepperstone, …): downstream consumer nodes that
consume the position-event table plus prices, apply real spread / commission /
slippage / lot / margin, may reject or modify positions, and produce a
**currency** equity stream for viability and deployment. The position-event table
stays broker-independent — one table feeds many realistic brokers, giving
directly comparable currency curves. Live: a realistic broker node consumes the
events in real time and routes orders as a side effect; reconciliation with the
real account is an external adapter.

So **the two broker classes attach at two different points**: the sim-optimal
broker on the **exposure stream** (signal quality, pips, now), realistic brokers
on the **derived position-event table** (execution viability, currency, later).
Both are ordinary downstream nodes (C8/C9); several can attach at once for
directly comparable curves.
**Forbids.** Treating an equity curve as the strategy's output; making the
**position-event table the strategy's direct DAG output** (it is derived, not
emitted per `eval` — a decision instant may need >1 event, which C8 forbids) or
the measure of signal quality; baking a broker into the strategy; a special
external broker subsystem (a broker is an ordinary node); storing `open_ts`
(derive it from the opening event); a signed-volume direction trick in the event
table (use `action`); broker-specific assumptions leaking into the strategy logic.
**Why.** The DAG is a synchronous reactive graph: at time t it holds exactly one
state, and a node emits at most one record per `eval` (C8). A *sequence of
position events* — where one decision instant (a stop-and-reverse) needs a close
**and** an open at the same `event_ts` — cannot be the DAG's per-cycle output
without violating C8. The state the DAG *can* express faithfully is the **desired
exposure** (one value per cycle); the position events are its first difference, a
derived consequence. Decoupling them resolves the C8↔C10 impedance and matches
how signal research actually proceeds: you first measure a signal's *quality*
(does this exposure, held over time, make pips?) independently of any execution
model; only later do you model position management and real-broker frictions for
deployment. The sim-optimal pip curve is a level, currency-free playing field for
comparing and combining signals; the derived position-event table + realistic
broker nodes then test real-world viability. Modelling brokers as nodes (not a
bespoke subsystem) keeps them within the one Node/graph abstraction (C9). This
supersedes the earlier "the strategy's output is the position-event table"
framing (cycle 0007 reframe).
**Realization (cycle 0007).** The signal-quality half of this contract is now
realized at the substrate, as two `aura-std` nodes composed on the unchanged
engine (the engine stays domain-free — it routes only `f64` records, never
"exposure" or "equity"). The **exposure stream** is realized as `Exposure { scale
}`: the decision/sizing node, `clamp(signal / scale, -1, +1)`, one `f64` per fired
cycle (`None` until warmed up). The **sim-optimal broker** is realized as
`SimBroker { pip_size }`: a two-input node (exposure, price) that accumulates
`prev_exposure · (price − prev_price) / pip_size` and emits cumulative pip equity
— the exposure held *into* a cycle (decided at t-1) earns that cycle's return, so
the integration is causal (C2), and `pip_size` is held reference metadata
(C7/C15), never streamed. An end-to-end harness (SMA-cross → `Exposure` →
`SimBroker` → recording sink) produces a recorded pip-equity curve, bit-identical
across runs (C1). The **position-management half** — deriving the position-event
table (buy/sell/close, `position_id`, partial closes) from the exposure history,
and the realistic broker nodes that consume it — is deliberately **not** built
this cycle; it remains the decoupled, derived, deferred layer described above.

### C11 — Generalized sources; record-then-replay determinism boundary
**Guarantee.** A source is anything that produces timestamped scalar streams —
market data (`data-server`) and non-financial sources (e.g. a news-agent node
emitting a bias) are treated identically. Anything nondeterministic, external,
or slow (LLM/news/web) is materialized into a recorded, timestamped stream
*before* it enters the engine; backtest replays the recording, live computes
fresh in real time and records it for future backtests. A bias enters as a value
held until the next event (firing policy A).
**Forbids.** Any live external call *inside* a backtest replay.
**Why.** It is the only model compatible with reproducible backtests — LLM calls
are nondeterministic and far too slow per-cycle. Per `~/.claude/CLAUDE.md`,
external LLM (IONOS) calls happen only at the recording/live-source edge, with
explicit per-session consent, never inside a sim.

### C12 — The atomic sim unit and the four orchestration axes
**Guarantee.** The atomic unit is `(frozen topology + param-set + data-window +
RNG-seed) → deterministic run → metrics`. Parameters are typed, ranged, runtime
values injected at graph build (no recompile per param-set; the optimizer sees a
generic vector of typed ranges). Raw data is shared read-only across sims via
`Arc<[T]>` (data-server is built for this). Four axes orchestrate the atomic
unit: (1) param-sweep (grid/random), (2) optimization (argmax metric),
(3) walk-forward (rolling in-sample optimize + out-of-sample test),
(4) Monte-Carlo (N seeded realizations perturbing input). **MC = sweep over
seeds**; each realization is itself deterministic given its seed.
*Status (cycle 0011).* The first ingestion (`aura-ingest`) **eagerly
materializes** a data-server window into owned source streams rather than
sharing `data-server`'s `Arc<[T]>` chunks zero-copy across sims — the cross-sim
`Arc<[T]>` sharing has no consumer until the orchestration axes above are built.
That sharing remains the target for the orchestration cycle that introduces
them; the transpose logic is unchanged by it. A known, deliberate gap, not drift.
**Forbids.** Baking a specific search strategy (Bayesian/genetic) into the
primitive — those are pluggable policies atop the atomic unit; recompiling on a
param change.
**Why.** A stable primitive + orchestration axes keeps "wahnsinnig schnell"
(embarrassingly parallel across the unit) cleanly separated from search policy.
Seed-as-input reconciles Monte-Carlo with C1. The "frozen topology" of the
atomic unit is one harness instance, selected by the harness's **structural
axes** (C20); the structural experiment matrix is the outer orchestration over
this dimension, the tuning sweep the inner (C19/C20).

### C13 — Hot-reload is authoring-only; deploy is frozen
**Guarantee.** A node/strategy is authored as a native Rust `cdylib`,
hot-reloaded during the authoring loop (Rust-ABI; host and node built with the
same toolchain). The live/deploy bot is a statically-linked, versioned, frozen
artifact.
**Forbids.** Hot-swapping a running live bot; loading third-party / foreign-
toolchain plugins.
**Why.** Hot-reload makes the research loop fast; a live artifact must be frozen
and reproducible (audit trail: this bot = this commit). A sweep pays no
hot-reload tax — params are runtime data (C12), so the cdylib loads once.

### C14 — Headless core, two faces
**Guarantee.** The engine is a UI-agnostic library. Two faces sit on it: a
**programmatic/CLI** face (the primary surface for the LLM and automation —
author a node, run a sim/sweep, emit structured metrics) and a **visual** face
for human exploration. Visualization is only a downstream consumer node on the
streams.
**Forbids.** Any UI/pixel knowledge inside the engine.
**Why.** The LLM drives programmatically, the human visually; a headless core
serves both. The visual face is the **playground** (C22) — egui-native,
in-process zero-copy from the SoA columns; it is staged after the runnable
substrate but is core to aura's identity, not optional.

### C15 — Resampling-as-node; sessions/calendars
**Guarantee.** A resampler is a node (finer stream → coarser bar stream),
clock-sensitive, emitting a completed bar only at the boundary (C2). Calendars
and instrument specs are metadata (non-scalar, beside the hot path); session
*context* is exposed as scalar streams via a `SessionNode` (`bars_since_open:
i64`, `in_session: bool`, `session_open_ts: timestamp`). "3rd 15m candle after
session open" is then a plain node checking `bars_since_open == 3`.
**Forbids.** Streaming the calendar; special-casing session logic outside the
stream model.
**Why.** Keeps the line consistent — everything a signal needs arrives as a
stream; reference data feeds source/session nodes from beside the hot path.

### C16 — Engine / project separation; three-tier node reuse
**Guarantee.** aura is the reusable **engine**; each research project is a
separate external repo that depends on aura via cargo (the game-engine / game
split). Node reuse is cargo-native, in three tiers: **`aura-std`** (universal
blocks, ship with the engine) / **shared node crates** (cross-project-reusable,
their own repos, pulled as cargo git deps) / **project-local `nodes/`**
(experimental, project-specific). A reusable node is an `rlib` dependency; the
hot-reload unit stays the project-side `cdylib` that composes it (consistent
with C13). Concretely a project is a **Rust crate** — a cdylib library of node /
strategy / experiment blueprints — plus a static `Aura.toml` (project context:
data paths, instrument/pip metadata, default broker & window, runs dir). During
research the `aura` host loads and runs it (C13 hot-reload); for deploy the
chosen strategy + broker freeze into a standalone binary. **A project is
therefore always a Rust program built on the engine.** Beyond the node-reuse
tiers, the engine workspace also carries non-node crates — `aura-engine` (the
run loop), `aura-cli` (the `aura` binary), and **`aura-ingest`** (the
data-source ingestion edge, cycle 0011, where the `data-server` external tree
enters). **Dependency policy (amended 2026-06-10, cycle 0029).** Dependencies
are admitted by deliberate, **per-case review** — what a crate pulls in weighed
against what it buys — with **particular scrutiny for anything that enters the
frozen deploy artifact** (C13: this bot = this commit). Well-established
standard crates (`serde`, `rayon`, …) pass that review and are used wherever
they do the job, including in the bot. There is **no blanket zero-dependency
commitment** and **no blanket admission**; hand-rolling what a vetted standard
crate already does is the anti-pattern, not the dependency. This strikes the
original *"zero-external-dependency by commitment"* clause and the
*"`aura-ingest` is the sole external-dependency firewall"* framing; `aura-ingest`
remains the data-source ingestion edge, no longer a dependency wall.
**Forbids.** Project-specific signals in the aura repo (it keeps at most
example/fixture nodes under `examples/` for its own tests); a multi-project
manager inside aura; a bespoke node registry/marketplace (cargo + Gitea *is* the
package mechanism). (Dependency admission is governed by the per-case policy
above, not a blanket ban.)
**Why.** The engine/game split keeps the engine sharp and reusable while each
project versions its own research with its own forward-queue. Promotion
(local → shared → std) is the ordinary Rust reuse gradient, no new mechanism.

### C17 — Authoring surface
**Guarantee.** All *logic* — nodes, strategies, **and experiments/harnesses** —
is authored in native Rust through **Claude Code + the skills pipeline**: the
human describes, Claude writes the Rust, builds it, runs it via the `aura` CLI,
and reports metrics. Declarative config (`Aura.toml`) carries only **static
project context** (data paths, instrument/pip metadata, defaults, runs dir),
never logic. aura ships **no embedded coding-LLM**. IONOS LLMs are used only as a
*runtime data source* (news-agent bias, C11), gated by per-session consent,
never in the code path.
**Forbids.** An in-app LLM chat that generates node code inside aura; using
IONOS (weaker models) as the authoring brain.
**Why.** LLMs author Rust well in Claude Code — that is the fix to RustAst's
failure; making weaker models the coding brain reintroduces the very problem.
Keeps aura's scope an engine + playground, not an LLM-IDE.

### C18 — Project management: one repo = one project, plus a run registry
**Guarantee.** Management has two planes. (1) **Code & forward-queue:** git
(commit = identity; the frozen bot *is* a commit) + Gitea (ideas/hypotheses as
the forward-queue, a research thrust = a milestone, the
`idea → experimental → validated → deployed` label gradient). (2) **Experiments
& results:** an Aura-native **run registry** — one record per run = a *manifest*
(node-commit + params + data-window + seed + broker profile) + *metrics*,
queryable, with *lineage* (composite ← signals; run ← inputs). Determinism
(C1/C12) makes a run reproducible from its tiny manifest, so the registry stores
manifests + metrics and re-derives full results on demand. Depth: **structured**
(promotion/status, lineage, run-diff).
**Forbids.** Storing results not reproducible from a recorded manifest;
duplicating git/Gitea inside aura; a multi-project workspace manager.
**Why.** Comparing experiments over time is the heart of the research loop and
has no home in git/Gitea; determinism makes a structured registry cheap.
Sequencing: the walking skeleton emits a manifest + metrics per run from day
one; the registry/index is a later milestone over manifests that already exist.

### C19 — Bootstrap: blueprint → instance (recursive)
**Guarantee.** Construction is a distinct phase, recursive at every level. Each
node type has a **factory** `params → sized concrete node` (e.g. `SMA(length)`
sizes its ring buffer). A **blueprint** is the param-generic, input-role-generic
graph-as-data produced by running a Rust builder (C9); it carries *free* numeric
params (declared ranges) and *free* input roles. The **bootstrap** binds
`(blueprint + param-set + data bindings + seed)` into a concrete, **frozen
instance** — buffers sized, topology fixed. This is precisely the "wiring /
graph build" that C7 ("sized at wiring", "topology frozen per sim") and C12
("params injected at graph build") already reference. The same machinery applies
recursively up to the harness (C20). A sweep builds many instances from one
blueprint; instances are disjoint (C1).
**Forbids.** Params that change topology (a topology change is a *different*
blueprint — Fork A, C7 "frozen"); resizing buffers after bootstrap; running a
sim against an un-bootstrapped blueprint.
**Why.** Separating the param-generic blueprint from the param-bound instance is
what makes one strategy reusable across a whole sweep and lets the optimizer
mutate "the 20" by *rebuilding* an instance (cheap; no recompile, C12) instead
of rewriting code. Naming the build phase makes the implicit "wiring" of C7/C12
explicit.
**Refinement (Construction-layer milestone, 2026-06-05).** This binding is a
**compilation**: the param-generic, named blueprint (source) is lowered to a flat,
type-erased **compilat** (C7) **wired by raw index, not by name**
(`Edge { from, to, slot, from_field }`): composite boundaries dissolve entirely,
and field / role names are demoted to **non-load-bearing** debug symbols (as
`FieldSpec.name` already is). "No recompile" above means no **Rust /
cdylib** rebuild (C12/C13: the cdylib loads once); re-deriving an instance per
param-set is a cheap **graph re-compilation**, not a code recompile. Naming the
build phase a compilation makes its successor explicit: the compilat is the target
of behaviour-preserving optimisation (C23).
**Realization (cycle 0016 — param-set injection).** The bootstrap now binds an
injected param-set, realizing C12's "params injected at graph build (the optimizer
sees a generic vector of typed ranges)" and C19's "factory `params → sized node`"
*literally*: a blueprint leaf is **value-empty** — `BlueprintNode::Leaf` holds a
`LeafFactory { name, params, build }` recipe, not a built node — and the value lives
only in the injected vector (no baked default), so the blueprint stays a pure
param-generic recipe. (Renamed in cycle 0024: `BlueprintNode::Primitive` holds a
`PrimitiveBuilder { name, schema, build }` — the recipe now carries the full
signature, see the C8 0024 realization; `bootstrap_with_params`/`compile_with_params`
moved onto `Composite` when `struct Blueprint` collapsed into it, see the C19 0024
realization.) `bootstrap_with_params(Vec<Scalar>)` /
`compile_with_params` **build each leaf through its own constructor** (the single
sizing/validation gate) from its kind-checked slice **while lowering** (build-then-
wire), consuming the vector slot-by-slot in the *same* depth-first walk
`param_space()` projects — so the two share one traversal (subsuming the #34 dual-
traversal hazard) and the value reaches the node at the slot the sweep enumerates.
Arity is checked up front (`param_space().len()`); a wrong-kind or wrong-length
vector is a typed `CompileError::{ParamKindMismatch, ParamArity}` (the typed-value
check C8 deferred). The lowering/edge/source rewrite is structurally unchanged, so
the **compilat stays bit-identical** for a given point (C23, the correctness
invariant). The value *domain* (e.g. `length ≥ 1`) stays the constructor's own
`assert`; the search-range is still the run's (#32/C20). One value-empty leaf
detail for C22: the blueprint view (pre-run, param-generic) labels a leaf by **bare
type** (`PrimitiveBuilder::label`, was `LeafFactory::label`, → `[SMA]`) — the
value-bearing `SMA(2)` of C8's render-
label refinement now appears only in the *compiled* view (built nodes, `Node::label`).
**Realization (cycle 0017 — blueprint render = main graph + definitions).** The
`aura graph` blueprint view (C9 graph-as-data, #13) renders the **authored
structure** as a *program with subroutines*: a flat **main graph** wiring the
harness with each composite shown as a **single opaque node** `[name]`, plus a
`where:` section that defines each **distinct** composite type **once** (its
interior with named input-entry nodes and outputs folded onto their producers as
`name := …` bindings (render refined through cycles 0019–0022; originally
`[in:k]`/`[out]` port markers); deduped by `name()`, collected
recursively so nested composites are opaque nodes with their own definitions). This
supersedes #13's original cluster-box model and is the durable split this view
realizes: **blueprint = source** (composites as named subroutines, body once) vs.
**compiled = inlined machine form** (C23, boundary dissolved). The substantive cause
for retiring cluster boxes is a real renderer defect — `ascii-dag` 0.9.1's subgraph
level-centering rounds sibling x-positions with `/2`, overlapping wide sibling
labels (width/parity-sensitive, no config/padding dodge surviving unequal-width
siblings); the **flat** layout is collision-free, and both views now build flat
graphs only. The model also scales (blueprint size tracks top-level wiring, not
inlined node count) and removes #13's nested-composite `unimplemented!` (the
definitions pass recurses). The interactive *enter/focus* counterpart (a composite
collapsed to a navigable node) is the playground's, parked as a separate concern
(#37); the static CLI keeps the all-at-once definitions form (#38).

**Realization (cycle 0024 — the root is the fully-bound composite; the compilat is
a named type).** `struct Blueprint` is **deleted**: the root graph IS a `Composite`,
and `compile_with_params`/`bootstrap_with_params`/`param_space` are its methods.
What distinguished the root — its bound data sources — is now a property of its input
roles: `Role` carries `source: Option<ScalarKind>` (`None` = an open interior port,
wired by the enclosing graph; `Some(kind)` = a bound ingestion feed). A composite is
**runnable iff every root role is bound** (C3: sources bind at ingestion only); an
open root role is a compile-time `CompileError::UnboundRootRole`. So the "main graph"
is no longer a separate kind — only the composite all of whose roles are
source-bound, governed by the same conditions as any other node. Compilation now
targets a **named type**: `compile` validates structurally **pre-build** (via
`signature()`, no node constructed — an ill-typed wiring is caught before any build
closure fires) and emits `FlatGraph { nodes, signatures, sources, edges }` — the
C23 compilat, now first-class — which `Harness::bootstrap` consumes (kinds/firing
from the carried signatures, buffer depth from `lookbacks()`). The per-flat-node
signature travels beside the node, so `bootstrap` reads it without a built-node
`schema()` call (which no longer exists, see the C8 0024 realization).

**Realization (cycle 0026 — graph render redesign: model + WASM-Graphviz viewer,
#51).** `aura graph` no longer renders ASCII. The render path is now two pieces:
a read-only **model serializer** (`aura_engine::model_to_json`, iteration 1) that
walks the root composite + every distinct composite type into a deterministic,
hand-rolled JSON model (C14, golden-tested like `RunReport::to_json`; the
swapped-param mis-wire property moved here from the old compiled-view test), and a
**self-contained HTML viewer** (`aura-cli::render::render_html`, iteration 2) that
inlines that model, the ported prototype viewer JS, and a vendored Graphviz-WASM
blob into one page emitted to stdout. Layout/SVG happen in the browser via
WebAssembly — aura ships no layout engine and stays a serializer (C9: graph-as-data,
no `eval`/build on the path). The viewer is a render asset (C10 — no node/strategy
logic, no DSL); it labels every input pin from the model's now-real names (C23
debug symbols, named in cycle 0027) and colours wires by the four scalar base
types (C4). This **retires `ascii-dag`** and its adapter (`graph.rs`), the
`--compiled`/`--macd` flag plumbing, and the invented `#Sf`/`:=`/`histogram →`
notation — superseding the cycle-0017 flat-ascii model and its renderer-defect
workaround. The DOT/SVG are Graphviz-version-dependent and not golden-tested; the
deterministic JSON model is the asserted contract.

### C20 — Strategy ↔ harness; the harness is the root sim graph
**Guarantee.** A **strategy** is a reusable composite-node blueprint (C9):
broker-, data-, and viz-independent, with inputs declared as named **roles**
(symbol-agnostic where possible) and the intent/exposure stream (C10) as output.
A **harness** (the experimental setup) is the **root sim graph** — sources bound
to the strategy's input roles + the strategy + attached broker node(s) + sinks —
and is itself produced by the bootstrap (C19). A harness *instance* is C1's
disjoint unit (RustAst's "root scope"). The harness has **two kinds of
parameterization**: **structural axes** (which strategy, which instrument(s),
which broker(s), which window) whose variation selects *different* instances —
the **experiment matrix**; and **tuning params** (the strategy's numeric params)
swept *within* a fixed structure (Fork A). The same strategy blueprint is reused
across backtest, sweep, visual workspaces, and the frozen live bot — each a
different harness. **Both strategy and harness/experiment are authored in Rust**
via builder APIs (C17); the experiment matrix is ordinary Rust control flow
(loops/conditionals), not a config schema. Ontologically, a node (incl. a
strategy composite) is an **open** fragment — free input roles + ≤1 output
(C8/C9) — that does not run alone; a **harness is the closed root graph**: a
strategy with its input roles bound to sources and its output terminated in
broker/sink nodes, under a clock. A harness is therefore **not a node** (no free
inputs, no output; it does not fit `eval`) — it is the *closure that runs*, C1's
disjoint unit / the root scope. Harnesses do not nest as nodes; the World (C21)
orchestrates them as objects.
**Forbids.** Embedding data sources / brokers / sinks inside a strategy; a
declarative experiment mini-DSL (logic is Rust — C17); modelling the harness as
a node (it is the closed root scope, not an open composable node).
**Why.** Reusability needs the strategy to be a context-free blueprint that many
harnesses embed. Modelling the harness as a root graph keeps it within the one
Node/graph abstraction (C9) and makes "10 strategies in one environment" and
"one strategy × N instruments" plain nested loops over the structural axes. Rust
authoring (not config) preserves full programmatic power — conditional/adaptive
matrices, generated axes, custom wiring — and avoids re-introducing the DSL trap
C17 rejects.

### C21 — The World: the meta-level is the product
**Guarantee.** Above the harness sits the **World** — the project's program /
"game" (C16: a Rust crate). Within it, **harnesses are dynamically constructible,
first-class objects**: meta-programs (walk-forward, sweep, optimize,
Monte-Carlo — C12's axes) construct harness instances at runtime via the
bootstrap (C19), run them disjointly in parallel (C1), aggregate / compare their
results, and discard the transient instances. A walk-forward rolls windows →
bootstraps a harness per window → stitches out-of-sample equity + parameter
stability into one meta-result. Orchestrating *families* of harnesses is
**first-class**, not a headless afterthought; the run registry (C18) is the
World's memory.
**Forbids.** Relegating multi-harness orchestration (walk-forward / sweep /
comparison) to second-class headless-only status; treating the single backtest
as the product.
**Why.** What happens *within* one harness — backtest a strategy → equity — is
commodity; every quant system covers it. aura exists for the meta-level: a
programmable space where harnesses are dynamically built and families of them
orchestrated and explored. The deterministic single-harness engine (C1–C20) is
the **substrate**; the World is the **product**.

### C22 — The playground is a trace explorer; sinks are the recording mechanism
**Guarantee.** The World is a *program*: nothing is displayable until it runs,
and its harnesses are transient machinery (built, run, discarded — C21). The only
durable, displayable substance is the **recorded trace**, captured by **sinks** —
pure consumer nodes (C8) that persist a stream (equity, position events, a node's
output) into the run registry (C18). **Displayable = exactly what a sink
recorded**; with no sink, only input params + summary metrics remain. The
**playground** is therefore an **execution viewer / trace explorer**, and it
plays *any* harness (it is the harness *player*, not a harness, never bound to a
default one): before a run it shows the program *structure* (graph-as-data, C9) +
param knobs; during a run, live sink streams; after a run, recorded traces +
metrics from the registry — including meta-views (stitched walk-forward, sweep
surfaces, multi-strategy / instrument comparison). Observability is **explicit
and selective** — you instrument what you want to see; the choice of sinks is
part of the experiment. The engine ships sample harnesses *with* sinks so a
newcomer sees a populated trace immediately; a fresh project is empty until
built, run, and instrumented.
**Forbids.** A scene-editor model that assumes a persistent populated world;
constructing or wiring topology in the UI (topology is Rust + hot-reload,
C9/C17 — the UI reflects the live graph-as-data and tunes runtime params via
sliders, C12); retaining un-sinked data; binding the playground to a single /
default harness.
**Why.** A program has no scene-at-rest to inspect; what persists is what it
records. Making sinks the one recording-and-observability mechanism keeps "what
can I see?" answerable by "what did I instrument?", and keeps the engine
UI-agnostic (C14). Live param tuning (runtime values, no topology change —
C12 / C19 Fork A) gives the interactive feel without a wiring DSL.
**Realization (cycle 0006).** Sinks-as-recording-mechanism is realized at the
substrate level: a recorded trace is exactly what a recording node pushed out of
the graph (no engine recording registry; the constructing World holds each
recording node's destination). The engine's single `observe: usize` affordance is
removed — `Harness::run` returns `()` and recording is a node-side concern, so one
run records *many* streams (one per recording node) instead of exactly one row.
Recorded streams are sparse and timestamped (a record per fired cycle, tagged
`ctx.now()`), matching a trace of timestamped events (C18). No new contract; the
`Harness` API change (observe removed, `run -> ()`) is recorded here.

### C23 — Graph compilation and behaviour-preserving optimisation
**Guarantee.** The bootstrap (C19) is a **compilation**: it lowers a param-generic,
named **blueprint** (the authoring source — nodes, composites, strategy, harness;
C8/C9/C20) into a flat, type-erased **compilat** — the frozen runnable
instance (C7) — **wired by raw index, not by name** (`Edge { from, to, slot,
from_field }`). Composites are **inlined** at this step (C9): the composite
*boundary* dissolves entirely (there is no composite in the compilat). The
blueprint's field / input-role *names* are **non-load-bearing** — the wiring
resolves by index, and names survive at most as **informative debug symbols**
(exactly as `FieldSpec.name` already is, C8), kept for tracing / rendering (C9
graph-as-data, #13) but carrying no run semantics. The compilat is then the target of **behaviour-preserving
optimisation** — any transform that leaves every observable sink trace
bit-identical (C1 is the correctness invariant) — on two levels:
- **intra-compilat** (within one graph): **common-subexpression elimination** —
  two identical nodes (same type, same bound params, same input source) merge to
  one with output fan-out, so fractal composition (C9) costs no redundant compute —
  and **dead-node elimination** — a node on no path to a sink is dropped.
  Pure-consumer sinks (C8, no output) are never CSE candidates, so their
  out-of-graph side effects are preserved by construction.
- **across the sweep family** (over the family of compilats one blueprint yields
  under a param sweep, C12): the sub-graph whose nodes carry **no swept param** and
  whose inputs are **all sweep-invariant** (transitively from the sources — same
  data window) is **loop-invariant** w.r.t. the sweep and is computed **once**; its
  output is materialised as a recorded stream (C11) shared read-only across the
  sweep instances (`Arc<[T]>`, C12). Only the parameter-dependent suffix re-runs
  per sweep point — loop-invariant code motion over the sweep loop.
**Forbids.** Any "optimisation" that changes an observable trace (it breaks C1);
optimising over a representation that is not graph-as-data — an opaque
trait-object interior (the rejected nested-composite reading of C9) cannot be
analysed or rewritten across its boundary, so it forecloses both levels.
**Why.** Determinism (C1) is not merely an audit property; it is the **licence**
for these rewrites — a behaviour-preserving transform is only meaningful because
"same input → bit-identical run" makes "same result" decidable, and a
sweep-invariant sub-graph is reproducible enough to compute once and share. The
compilat representation (C19) is the necessary condition: only a flat, inspectable
graph-as-data — with each node's declared param-ranges (C8) — lets the compiler
identify identical sub-expressions, dead nodes, and the sweep-invariant frontier.
This is precisely why a composite is an inlining (C9) and not a runtime
sub-engine: the flat compilat is what makes the whole graph optimisable.
**Status (Construction-layer milestone).** The compilat *representation* —
blueprint → inline / compile → flat instance — is the load-bearing substrate built
**first**; the optimisation passes (intra-compilat CSE/DCE, sweep-invariant
hoisting) are **deferred**, behaviour-preserving follow-on work, each gated by a
"compilat with pass ≡ compilat without pass, bit-identical run" test (C1). The
sweep-level pass further presupposes per-node param declarations (C8 — landed in
cycle 0015 as `name`+`kind`; the swept *range* still pending #32/C20) and the sweep
orchestration itself (C12/C21).

---

## Open architectural threads not yet resolved

- **Playground & World UI surface** — the playground is core (C22), egui-native;
  open is the concrete UI of the meta-views (walk-forward, sweep surfaces,
  comparison) and the run/replay clock controls.
- **Parameter-space search strategies** (Bayesian/genetic) — pluggable policies
  atop the atomic sim unit (C12), not yet designed.
- **`aura new` scaffolder, the experiment-builder API, and `Aura.toml`'s
  static-context schema** — `aura new` scaffolds a Rust project *crate* (node /
  strategy / experiment blueprints) against the engine (C16/C20); the
  experiment-builder API surface (harness wiring, structural axes, sweep
  combinators) and `Aura.toml`'s schema (data paths, instrument/pip metadata,
  default broker & window, runs dir) are not yet designed.
- **`aura-std` contents** — the crate exists (doc-only); which universal blocks
  land first follows the walking-skeleton's needs.
- **`strategies/` split** — a later split, *inside a project*, of top-level
  strategies from reusable building blocks in `nodes/`; not a day-1 cut.
- **Sequencing** — the runnable single-harness *substrate* comes first (walking
  skeleton: a closed harness that runs deterministically and records via a sink);
  the World/meta layer (C21) and the playground trace-explorer (C22) are the
  differentiating layers that follow — you orchestrate and visualize a thing that
  must first run once.