docs+scaffold: engine/project split (C16-C18), aura-std, usage doc

Refine the structure per interview: aura is the reusable engine, research projects are separate external repos depending on it (C16). Add aura-std (universal standard-node tier) to the workspace; remove nodes/ from the engine (project concept). Record authoring-surface = Claude Code + skills pipeline, no embedded coding-LLM (C17), and project management = one-repo-one-project + Aura-native run registry (C18). Add CLAUDE.md invariants 9-10, code_roots -> [crates], and docs/project-layout.md documenting the day-in-the-life and project repo layout.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/project-layout.md

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+# How aura is used — engine, projects, and a day in the life
+
+This document describes the *outside* of aura: how you sit in front of it and
+what a research session looks like. The *inside* (the engine's contracts) is the
+design ledger (`docs/design/INDEX.md`).
+
+## Engine vs. project (the game-engine split)
+
+aura is a **game engine for traders**, and like any game engine it is separate
+from the things built with it:
+
+- **This repo (`aura`)** is the **engine**: `aura-core` (the shared contract),
+  `aura-std` (universal standard nodes), `aura-engine` (the deterministic sim
+  runtime), `aura-cli` (the `aura` binary), and later the egui playground. It
+  ships at most `examples/` fixture nodes for its own tests — never your real
+  signals.
+- **A research project** is its **own external repo** (e.g. `~/dev/ger40-lab/`),
+  with its own git history, its own Gitea repo (its own forward-queue), and a
+  cargo dependency on aura. This is where you and Claude author signals and where
+  the runs live. (See contract **C16**.)
+
+The `aura` CLI is a tool you run *inside* a project directory — it finds the
+project root by walking up to an `Aura.toml`, the way `cargo` finds `Cargo.toml`.
+
+## A project repo
+
+```
+ger40-lab/                 # your research project — a separate repo
+├── Aura.toml              # project config: symbols in scope, default data
+│                          # window, broker profile, runs dir  (schema TBD)
+├── Cargo.toml             # depends on aura-core (+ shared node crates)
+├── CLAUDE.md              # the project's own skills wiring (authoring discipline)
+├── .claude/dev-cycle-profile.yml
+├── nodes/                 # project-local, experimental nodes (cdylib)
+│   └── third-candle-long/
+├── runs/                  # the run registry: manifest + metrics per run
+└── (frozen bots, results, …)
+```
+
+## Where reusable nodes live (three tiers)
+
+Everything that plugs into the engine is fractally a `Node`. Reuse is plain
+cargo, layered by maturity (contract C16):
+
+1. **`aura-std`** — universal blocks shipped with the engine (SMA/ATR/RSI,
+   resamplers, the `SessionNode`, standard combinators, broker profiles).
+2. **Shared node crates** — cross-project-reusable blocks in their own repos
+   (e.g. `Brummel/aura-nodes-fx`), pulled in as cargo git dependencies.
+3. **Project-local `nodes/`** — experimental, project-specific blocks.
+
+Promotion is the ordinary Rust gradient: a node proves itself project-local →
+gets extracted into a shared crate → if it turns universal, into `aura-std`.
+Shared nodes are `rlib` dependencies; the hot-reload unit stays the project-side
+`cdylib` that composes them, so editing a shared node still rebuilds and reloads
+the dependent.
+
+## Authoring happens in Claude Code (contract C17)
+
+aura has no built-in coding-LLM. You author by talking to Claude Code, which
+writes the node crate, builds it, runs it, and reports back. IONOS LLMs appear
+only as a *runtime data source* (e.g. a news-agent node emitting a bias),
+recorded before it enters a backtest, and only with your per-session consent.
+
+## A day in the life
+
+1. **You** (in Claude Code, inside `ger40-lab/`): "I suspect the 3rd 15m candle
+   after GER40 open is long-biased when the first two close bullish — build it as
+   a signal."
+2. **Claude** (via the skills pipeline) writes `nodes/third-candle-long/`,
+   implements `schema` + `eval` against `aura-core`.
+3. **Backtest:** `aura backtest nodes/third-candle-long --symbol GER40
+   --from 2020 --to 2024 --broker pepperstone` → a metrics table (trades,
+   hit-rate, P&L, max-DD, Sharpe) + a run record (manifest + metrics) under
+   `runs/`.
+4. **Sweep + Monte-Carlo:** `aura sweep … --grid lookback=5..50,thresh=0.5..0.9
+   --mc-seeds 1000` → a distribution of metrics, best params, robustness bands.
+5. **Compose:** "combine it with `momentum-filter` as a weighted sum" → Claude
+   writes a composite node (fractal, C9).
+6. **Walk-forward:** `aura walkforward …` → an out-of-sample verdict.
+7. **Freeze:** `aura freeze nodes/strategy-y --broker pepperstone
+   --out bots/strategy-y` → a standalone, statically-linked bot (C13).
+8. **(Later) Look:** `aura play` opens the egui playground — equity curve, trade
+   overlays on the chart, sweep heatmaps. For *looking*, not authoring (C14).
+
+CLI command names and `Aura.toml` are illustrative; the concrete surface is
+designed with the relevant milestone (see the open threads in the design ledger).
+The forward-queue — what to try next — lives in the project's Gitea tracker, not
+in code (contract C18).