plan: 0012 blueprint-compile composites

Bite-sized plan for the construction layer: Blueprint/Composite types + derived schema (Task 1), the recursive compile() inliner with typed CompileError validation (Task 2), and the SMA-cross bit-identical demonstrator (Task 3). Lowers to the unchanged Harness::bootstrap; optimisation passes deferred (C23). refs #12
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+# Blueprint → compilat: composite inlining — Implementation Plan
+
+> **Parent spec:** `docs/specs/0012-blueprint-compile-composites.md`
+>
+> **For agentic workers:** REQUIRED SUB-SKILL: use the `implement` skill to run
+> this plan. Steps use `- [ ]` checkboxes for tracking.
+
+**Goal:** Add a `Blueprint` / `Composite` construction layer to `aura-engine` that
+compiles a named graph-as-data — inlining composites by raw-index lowering — into
+the flat `(nodes, sources, edges)` the unchanged `Harness::bootstrap` consumes, and
+prove an SMA-cross composite runs bit-identically to today's hand-wired graph (C1).
+
+**Architecture:** A new module `crates/aura-engine/src/blueprint.rs` sits *above*
+`Harness::bootstrap`. `Blueprint::compile()` recursively inlines every `Composite`
+(append interior nodes at an offset, rewrite interior edges, fan input roles out,
+resolve the one output port), producing the same flat compilat a hand-wiring would.
+The run loop, `bootstrap`'s signature, and `Edge`/`Target`/`SourceSpec`/`Node` are
+untouched; bootstrap's existing kind- and Kahn-cycle-check validate the lowered
+compilat. Optimisation passes (C23) are explicitly out of scope.
+
+**Tech Stack:** Rust, `aura-engine` (depends on `aura-core`; `aura-std` is a
+dev-dependency reachable from tests). No new external dependencies (C16).
+
+---
+
+**Files this plan creates or modifies:**
+
+- Create: `crates/aura-engine/src/blueprint.rs` — the construction layer:
+  `OutPort`, `BlueprintNode` (+ `From<N: Node>` lift), `Composite` (`new` + derived
+  `schema`), `Blueprint` (`new` + `compile` + `bootstrap`), `CompileError`, the
+  recursive inliner, and an inline `#[cfg(test)] mod tests`.
+- Modify: `crates/aura-engine/src/lib.rs:34-38` — declare `mod blueprint;` and
+  re-export the public construction types.
+- Test: `crates/aura-engine/src/blueprint.rs` (inline `#[cfg(test)] mod tests`) —
+  schema derivation, inliner happy-path + nested + error paths, and the headline
+  bit-identical demonstrator `composite_sma_cross_runs_bit_identical_to_hand_wired`.
+
+Reference shapes (read-only, must NOT change): `crates/aura-engine/src/harness.rs`
+(`Edge`/`Target`/`SourceSpec` `:29-52`, `BootstrapError` `:56-65`,
+`Harness::bootstrap` `:114-200`, run loop `:208-283`), `crates/aura-core/src/node.rs`
+(`Node`/`NodeSchema`/`InputSpec`/`FieldSpec`/`Firing`), `crates/aura-cli/src/main.rs:42-78`
+(the `sample_harness` wiring the demonstrator reproduces).
+
+---
+
+### Task 1: Construction-layer types + derived schema
+
+**Files:**
+- Create: `crates/aura-engine/src/blueprint.rs`
+- Modify: `crates/aura-engine/src/lib.rs:34-38`
+- Test: `crates/aura-engine/src/blueprint.rs` (inline `mod tests`)
+
+- [ ] **Step 1: Create `blueprint.rs` with the types, the `From` lift, and derived schema**
+
+Create `crates/aura-engine/src/blueprint.rs` with exactly this content:
+
+```rust
+//! The construction layer (C9/C19/C23): a named, param-generic graph-as-data
+//! (`Blueprint`) that **compiles** to the flat, type-erased instance the run loop
+//! already runs (the *compilat*). The unit of reuse is the [`Composite`]: a
+//! nestable sub-graph fragment exposing one output port (C8) and named input
+//! roles, which `compile` **inlines** into the flat `(nodes, sources, edges)` the
+//! unchanged [`crate::Harness::bootstrap`] consumes.
+//!
+//! The compilat is wired by raw index, **not by name** (C23): a composite's
+//! boundary dissolves at compile time; field/role names, where kept, are
+//! non-load-bearing debug symbols (as `FieldSpec.name` already is). This module
+//! adds no optimisation pass (CSE/DCE, sweep-invariant hoisting are deferred,
+//! C23) and no external dependency (C16).
+
+use aura_core::{Node, NodeSchema, ScalarKind};
+
+use crate::harness::{BootstrapError, Edge, Harness, SourceSpec, Target};
+
+/// Which interior `(node, output-field)` is a composite's single output port (C8).
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct OutPort {
+    pub node: usize,
+    pub field: usize,
+}
+
+/// A blueprint item: a leaf node or a nested composite. Both present a declared
+/// interface (typed inputs + one output) to the enclosing graph.
+pub enum BlueprintNode {
+    Leaf(Box<dyn Node>),
+    Composite(Composite),
+}
+
+/// Ergonomic lift: any concrete `Node` becomes a `Leaf` blueprint item.
+impl<N: Node + 'static> From<N> for BlueprintNode {
+    fn from(node: N) -> Self {
+        BlueprintNode::Leaf(Box::new(node))
+    }
+}
+
+impl BlueprintNode {
+    /// The declared interface this item presents to the enclosing graph: a leaf's
+    /// own `Node::schema`, or a composite's derived [`Composite::schema`].
+    fn schema(&self) -> NodeSchema {
+        match self {
+            BlueprintNode::Leaf(node) => node.schema(),
+            BlueprintNode::Composite(c) => c.schema(),
+        }
+    }
+}
+
+/// A reusable sub-graph fragment compiled away by inlining (C9/C23). It is **not**
+/// a [`Node`]: it is never `eval`'d. It holds interior items (local indices),
+/// interior edges (local indices), input roles (role `r` fans into the interior
+/// targets `input_roles[r]`), and the one exposed output port.
+pub struct Composite {
+    nodes: Vec<BlueprintNode>,
+    edges: Vec<Edge>,
+    input_roles: Vec<Vec<Target>>,
+    output: OutPort,
+}
+
+impl Composite {
+    /// Build a composite from its interior items, interior edges (local indices),
+    /// input roles, and output port.
+    pub fn new(
+        nodes: Vec<BlueprintNode>,
+        edges: Vec<Edge>,
+        input_roles: Vec<Vec<Target>>,
+        output: OutPort,
+    ) -> Self {
+        Self { nodes, edges, input_roles, output }
+    }
+
+    /// The derived interface the enclosing graph wires against: input role `r`'s
+    /// spec is taken from its first interior target's slot; the output field is the
+    /// interior output port's field. This is a *derivation*, not a `Node` impl, and
+    /// it assumes well-formed indices — `compile` is the validator that rejects a
+    /// malformed composite with a typed [`CompileError`].
+    pub fn schema(&self) -> NodeSchema {
+        let inputs = self
+            .input_roles
+            .iter()
+            .map(|role| {
+                let first = role[0];
+                self.nodes[first.node].schema().inputs[first.slot]
+            })
+            .collect();
+        let out_field = self.nodes[self.output.node].schema().output[self.output.field];
+        NodeSchema { inputs, output: vec![out_field] }
+    }
+}
+
+/// The root graph-as-data, before compilation: blueprint items + sources + edges,
+/// all addressing blueprint-level indices.
+pub struct Blueprint {
+    nodes: Vec<BlueprintNode>,
+    sources: Vec<SourceSpec>,
+    edges: Vec<Edge>,
+}
+
+impl Blueprint {
+    /// Build a blueprint from its items, sources, and edges (blueprint-level
+    /// indices; a target/edge endpoint may name a composite).
+    pub fn new(nodes: Vec<BlueprintNode>, sources: Vec<SourceSpec>, edges: Vec<Edge>) -> Self {
+        Self { nodes, sources, edges }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use aura_core::{Ctx, FieldSpec, Firing, InputSpec, Scalar};
+
+    /// A 2-input f64 node, one f64 output. Test-local fixture (C9: examples for the
+    /// engine's own tests, no speculative `aura-std` surface).
+    struct Join2 {
+        out: [Scalar; 1],
+    }
+    impl Node for Join2 {
+        fn schema(&self) -> NodeSchema {
+            NodeSchema {
+                inputs: vec![
+                    InputSpec { kind: ScalarKind::F64, lookback: 1, firing: Firing::Any },
+                    InputSpec { kind: ScalarKind::F64, lookback: 1, firing: Firing::Any },
+                ],
+                output: vec![FieldSpec { name: "v", kind: ScalarKind::F64 }],
+            }
+        }
+        fn eval(&mut self, ctx: Ctx<'_>) -> Option<&[Scalar]> {
+            let a = ctx.f64_in(0);
+            let b = ctx.f64_in(1);
+            if a.is_empty() || b.is_empty() {
+                return None;
+            }
+            self.out[0] = Scalar::F64(a[0] + b[0]);
+            Some(&self.out)
+        }
+    }
+
+    #[test]
+    fn composite_schema_derives_role_and_output_kinds() {
+        // one interior node (Join2: 2 f64 inputs, 1 f64 output); two roles, each
+        // feeding one interior slot; output port = the Join2 output field 0.
+        let c = Composite::new(
+            vec![BlueprintNode::Leaf(Box::new(Join2 { out: [Scalar::F64(0.0)] }))],
+            vec![],
+            vec![
+                vec![Target { node: 0, slot: 0 }],
+                vec![Target { node: 0, slot: 1 }],
+            ],
+            OutPort { node: 0, field: 0 },
+        );
+        let schema = c.schema();
+        assert_eq!(schema.inputs.len(), 2);
+        assert_eq!(schema.inputs[0].kind, ScalarKind::F64);
+        assert_eq!(schema.inputs[1].kind, ScalarKind::F64);
+        assert_eq!(schema.output, vec![FieldSpec { name: "v", kind: ScalarKind::F64 }]);
+    }
+}
+```
+
+- [ ] **Step 2: Declare the module and re-export the public types in `lib.rs`**
+
+In `crates/aura-engine/src/lib.rs`, replace the module declarations and re-export
+block (`:34-38`):
+
+```rust
+mod harness;
+mod report;
+```
+
+becomes:
+
+```rust
+mod blueprint;
+mod harness;
+mod report;
+```
+
+and add a re-export line after the existing `pub use harness::{...};` (`:37`) so the
+block reads:
+
+```rust
+pub use blueprint::{Blueprint, BlueprintNode, Composite, OutPort};
+pub use harness::{BootstrapError, Edge, Harness, SourceSpec, Target};
+pub use report::{f64_field, summarize, RunManifest, RunMetrics, RunReport};
+```
+
+- [ ] **Step 3: Run the schema test to verify it passes**
+
+Run: `cargo test -p aura-engine blueprint::tests::composite_schema_derives_role_and_output_kinds`
+Expected: PASS (`test result: ok. 1 passed`).
+
+- [ ] **Step 4: Verify the workspace still compiles and existing tests stay green**
+
+Run: `cargo test -p aura-engine`
+Expected: PASS — the existing harness/report tests still pass and the one new
+schema test passes; `0 failed`.
+
+---
+
+### Task 2: The recursive inliner — `compile()` + `bootstrap()`
+
+**Files:**
+- Modify: `crates/aura-engine/src/blueprint.rs` (add `CompileError`, the inliner,
+  and `impl Blueprint { compile, bootstrap }`)
+- Modify: `crates/aura-engine/src/lib.rs` (add `CompileError` to the re-export)
+- Test: `crates/aura-engine/src/blueprint.rs` (inline `mod tests`)
+
+- [ ] **Step 1: Write the failing inliner tests**
+
+Append these test fixtures and tests inside the existing `#[cfg(test)] mod tests`
+in `crates/aura-engine/src/blueprint.rs` (after the `Join2` fixture and the schema
+test):
+
+```rust
+    /// A 1-input f64 node, one f64 output. Test-local fixture.
+    struct Pass1 {
+        out: [Scalar; 1],
+    }
+    impl Node for Pass1 {
+        fn schema(&self) -> NodeSchema {
+            NodeSchema {
+                inputs: vec![InputSpec { kind: ScalarKind::F64, lookback: 1, firing: Firing::Any }],
+                output: vec![FieldSpec { name: "v", kind: ScalarKind::F64 }],
+            }
+        }
+        fn eval(&mut self, ctx: Ctx<'_>) -> Option<&[Scalar]> {
+            let w = ctx.f64_in(0);
+            if w.is_empty() {
+                return None;
+            }
+            self.out[0] = Scalar::F64(w[0]);
+            Some(&self.out)
+        }
+    }
+
+    /// A pure consumer with one f64 input and no output (sink role, C8).
+    struct SinkF64;
+    impl Node for SinkF64 {
+        fn schema(&self) -> NodeSchema {
+            NodeSchema {
+                inputs: vec![InputSpec { kind: ScalarKind::F64, lookback: 1, firing: Firing::Any }],
+                output: vec![],
+            }
+        }
+        fn eval(&mut self, _ctx: Ctx<'_>) -> Option<&[Scalar]> {
+            None
+        }
+    }
+
+    /// A pure consumer with one i64 input and no output. Used to provoke a role /
+    /// edge kind mismatch (its slot is i64 where an f64 is fanned in).
+    struct SinkI64;
+    impl Node for SinkI64 {
+        fn schema(&self) -> NodeSchema {
+            NodeSchema {
+                inputs: vec![InputSpec { kind: ScalarKind::I64, lookback: 1, firing: Firing::Any }],
+                output: vec![],
+            }
+        }
+        fn eval(&mut self, _ctx: Ctx<'_>) -> Option<&[Scalar]> {
+            None
+        }
+    }
+
+    fn pass1() -> BlueprintNode {
+        BlueprintNode::Leaf(Box::new(Pass1 { out: [Scalar::F64(0.0)] }))
+    }
+    fn join2() -> BlueprintNode {
+        BlueprintNode::Leaf(Box::new(Join2 { out: [Scalar::F64(0.0)] }))
+    }
+
+    /// A composite: two Pass1 leaves feeding a Join2, role 0 fanning the source
+    /// into BOTH Pass1 slots, output = the Join2 field 0. The generic analogue of
+    /// the SMA-cross shape.
+    fn fan_composite() -> Composite {
+        Composite::new(
+            vec![pass1(), pass1(), join2()],
+            vec![
+                Edge { from: 0, to: 2, slot: 0, from_field: 0 },
+                Edge { from: 1, to: 2, slot: 1, from_field: 0 },
+            ],
+            vec![vec![Target { node: 0, slot: 0 }, Target { node: 1, slot: 0 }]],
+            OutPort { node: 2, field: 0 },
+        )
+    }
+
+    #[test]
+    fn single_composite_inlines_with_offset_fan_and_output() {
+        // composite as item 0; a source into its role 0; an edge out of it to a sink.
+        let bp = Blueprint::new(
+            vec![BlueprintNode::Composite(fan_composite()), BlueprintNode::Leaf(Box::new(SinkF64))],
+            vec![SourceSpec { kind: ScalarKind::F64, targets: vec![Target { node: 0, slot: 0 }] }],
+            vec![Edge { from: 0, to: 1, slot: 0, from_field: 0 }],
+        );
+        let (nodes, sources, edges) = bp.compile().expect("valid composite");
+
+        // 3 interior nodes (Pass1, Pass1, Join2) at flat 0..2, then SinkF64 at 3
+        assert_eq!(nodes.len(), 4);
+        // interior edges rewritten at offset 0, then the output edge resolves the
+        // composite's OutPort (interior node 2, field 0) to the sink (flat node 3)
+        assert_eq!(
+            edges,
+            vec![
+                Edge { from: 0, to: 2, slot: 0, from_field: 0 },
+                Edge { from: 1, to: 2, slot: 1, from_field: 0 },
+                Edge { from: 2, to: 3, slot: 0, from_field: 0 },
+            ]
+        );
+        // the source target into role 0 fanned into BOTH Pass1 slots
+        assert_eq!(sources.len(), 1);
+        assert_eq!(
+            sources[0].targets,
+            vec![Target { node: 0, slot: 0 }, Target { node: 1, slot: 0 }]
+        );
+    }
+
+    #[test]
+    fn nested_composite_inlines() {
+        // outer composite wraps the inner fan_composite as its only interior item,
+        // re-exposing the inner's role 0 (outer role 0 -> inner role 0) and the
+        // inner's output. A source into the outer role 0 must fan to BOTH inner
+        // Pass1 slots; the inner Join2 lands at flat index 2.
+        let inner = fan_composite();
+        let outer = Composite::new(
+            vec![BlueprintNode::Composite(inner)],
+            vec![],
+            vec![vec![Target { node: 0, slot: 0 }]],
+            OutPort { node: 0, field: 0 },
+        );
+        let bp = Blueprint::new(
+            vec![BlueprintNode::Composite(outer), BlueprintNode::Leaf(Box::new(SinkF64))],
+            vec![SourceSpec { kind: ScalarKind::F64, targets: vec![Target { node: 0, slot: 0 }] }],
+            vec![Edge { from: 0, to: 1, slot: 0, from_field: 0 }],
+        );
+        let (nodes, sources, edges) = bp.compile().expect("valid nested composite");
+
+        assert_eq!(nodes.len(), 4); // Pass1, Pass1, Join2, SinkF64
+        assert_eq!(
+            sources[0].targets,
+            vec![Target { node: 0, slot: 0 }, Target { node: 1, slot: 0 }]
+        );
+        // inner interior edges + the output edge from the inner Join2 (flat 2) to sink
+        assert_eq!(
+            edges,
+            vec![
+                Edge { from: 0, to: 2, slot: 0, from_field: 0 },
+                Edge { from: 1, to: 2, slot: 1, from_field: 0 },
+                Edge { from: 2, to: 3, slot: 0, from_field: 0 },
+            ]
+        );
+    }
+
+    #[test]
+    fn bad_interior_index_rejected() {
+        // interior edge references interior node 9, which does not exist
+        let c = Composite::new(
+            vec![pass1()],
+            vec![Edge { from: 0, to: 9, slot: 0, from_field: 0 }],
+            vec![vec![Target { node: 0, slot: 0 }]],
+            OutPort { node: 0, field: 0 },
+        );
+        let bp = Blueprint::new(vec![BlueprintNode::Composite(c)], vec![], vec![]);
+        assert_eq!(bp.compile().unwrap_err(), CompileError::BadInteriorIndex);
+    }
+
+    #[test]
+    fn role_kind_mismatch_rejected() {
+        // role 0 fans into a Pass1 f64 slot AND a SinkI64 i64 slot -> mismatch
+        let c = Composite::new(
+            vec![pass1(), BlueprintNode::Leaf(Box::new(SinkI64))],
+            vec![],
+            vec![vec![Target { node: 0, slot: 0 }, Target { node: 1, slot: 0 }]],
+            OutPort { node: 0, field: 0 },
+        );
+        let bp = Blueprint::new(vec![BlueprintNode::Composite(c)], vec![], vec![]);
+        assert_eq!(bp.compile().unwrap_err(), CompileError::RoleKindMismatch { role: 0 });
+    }
+
+    #[test]
+    fn output_port_out_of_range_rejected() {
+        // output names field 5 of a node whose output has one field
+        let c = Composite::new(
+            vec![pass1()],
+            vec![],
+            vec![vec![Target { node: 0, slot: 0 }]],
+            OutPort { node: 0, field: 5 },
+        );
+        let bp = Blueprint::new(vec![BlueprintNode::Composite(c)], vec![], vec![]);
+        assert_eq!(bp.compile().unwrap_err(), CompileError::OutputPortOutOfRange);
+    }
+
+    #[test]
+    fn bootstrap_error_is_wrapped() {
+        // a top-level kind mismatch: a Pass1 f64 output wired into a SinkI64 i64
+        // input. compile() lowers it faithfully; bootstrap's kind-check rejects it.
+        let bp = Blueprint::new(
+            vec![pass1(), BlueprintNode::Leaf(Box::new(SinkI64))],
+            vec![],
+            vec![Edge { from: 0, to: 1, slot: 0, from_field: 0 }],
+        );
+        match bp.bootstrap().unwrap_err() {
+            CompileError::Bootstrap(BootstrapError::KindMismatch { producer, consumer }) => {
+                assert_eq!(producer, ScalarKind::F64);
+                assert_eq!(consumer, ScalarKind::I64);
+            }
+            other => panic!("expected Bootstrap(KindMismatch), got {other:?}"),
+        }
+    }
+```
+
+- [ ] **Step 2: Run the inliner tests to verify they fail**
+
+Run: `cargo test -p aura-engine blueprint::tests`
+Expected: FAIL — compile error `no method named \`compile\` found` / `no method
+named \`bootstrap\`` / `cannot find type \`CompileError\`` (the inliner does not
+exist yet).
+
+- [ ] **Step 3: Add `CompileError`, the lowering helpers, and `impl Blueprint`**
+
+In `crates/aura-engine/src/blueprint.rs`, add the `CompileError` enum immediately
+after the `Composite` impl block (before `pub struct Blueprint`):
+
+```rust
+/// A construction-phase fault, caught before the flat compilat reaches
+/// `Harness::bootstrap`.
+#[derive(Debug, PartialEq, Eq)]
+pub enum CompileError {
+    /// An interior edge, role target, or output index is out of range.
+    BadInteriorIndex,
+    /// Input role `role` fans into interior slots of differing scalar kinds.
+    RoleKindMismatch { role: usize },
+    /// The output port names a missing interior node or output field.
+    OutputPortOutOfRange,
+    /// The lowered flat compilat failed `Harness::bootstrap`'s checks (kind
+    /// mismatch, bad index, or directed cycle).
+    Bootstrap(BootstrapError),
+}
+```
+
+Then add the `compile` and `bootstrap` methods inside `impl Blueprint` (after
+`new`):
+
+```rust
+    /// Lower to the flat compilat: inline every composite (recursive), offset
+    /// interior indices, rewrite edges, and fan input roles out. The run loop and
+    /// `bootstrap`'s data model are unchanged; the lowered compilat is wired by raw
+    /// index (C23).
+    // The flat triple is exactly `Harness::bootstrap`'s argument list; naming it
+    // would be a speculative type alias this cycle (same call as the CLI's sample).
+    #[allow(clippy::type_complexity)]
+    pub fn compile(self) -> Result<(Vec<Box<dyn Node>>, Vec<SourceSpec>, Vec<Edge>), CompileError> {
+        let mut flat_nodes: Vec<Box<dyn Node>> = Vec::new();
+        let mut flat_edges: Vec<Edge> = Vec::new();
+
+        // lower every top-level item (recursively inlining composites)
+        let lowerings = lower_items(self.nodes, &mut flat_nodes, &mut flat_edges)?;
+
+        // rewrite top-level edges through the lowerings (fan-out into composites)
+        for e in &self.edges {
+            for fe in rewrite_edge(e, &lowerings, &flat_nodes)? {
+                flat_edges.push(fe);
+            }
+        }
+
+        // rewrite sources: each target into a composite fans into its role targets
+        let mut flat_sources: Vec<SourceSpec> = Vec::with_capacity(self.sources.len());
+        for src in &self.sources {
+            let mut targets: Vec<Target> = Vec::new();
+            for t in &src.targets {
+                targets.extend(resolve_target(t, &lowerings)?);
+            }
+            flat_sources.push(SourceSpec { kind: src.kind, targets });
+        }
+
+        Ok((flat_nodes, flat_sources, flat_edges))
+    }
+
+    /// Compile, then hand the flat compilat to the unchanged `Harness::bootstrap`.
+    pub fn bootstrap(self) -> Result<Harness, CompileError> {
+        let (nodes, sources, edges) = self.compile()?;
+        Harness::bootstrap(nodes, sources, edges).map_err(CompileError::Bootstrap)
+    }
+```
+
+Finally add the free lowering helpers and the `ItemLowering` enum at the end of the
+file (after the `impl Blueprint` block, before `#[cfg(test)] mod tests`):
+
+```rust
+/// How one blueprint item resolved into the flat compilat. Edges and source
+/// targets to/from an item are resolved through this.
+enum ItemLowering {
+    /// A leaf lowered to exactly one flat node at this index.
+    Leaf { index: usize },
+    /// A composite lowered to its interior: its single output port is this flat
+    /// `(node, field)`, and input role `r` fans into `roles[r]` (flat targets).
+    Composite { output: (usize, usize), roles: Vec<Vec<Target>> },
+}
+
+/// Lower a list of blueprint items into the flat node array, appending interior
+/// nodes and (for composites) their interior edges. Returns one `ItemLowering` per
+/// input item, in order.
+fn lower_items(
+    items: Vec<BlueprintNode>,
+    flat_nodes: &mut Vec<Box<dyn Node>>,
+    flat_edges: &mut Vec<Edge>,
+) -> Result<Vec<ItemLowering>, CompileError> {
+    let mut lowerings = Vec::with_capacity(items.len());
+    for item in items {
+        match item {
+            BlueprintNode::Leaf(node) => {
+                let index = flat_nodes.len();
+                flat_nodes.push(node);
+                lowerings.push(ItemLowering::Leaf { index });
+            }
+            BlueprintNode::Composite(c) => {
+                lowerings.push(inline_composite(c, flat_nodes, flat_edges)?);
+            }
+        }
+    }
+    Ok(lowerings)
+}
+
+/// Inline one composite: recursively lower its interior items, rewrite its interior
+/// edges, then resolve its output port and per-role flat targets.
+fn inline_composite(
+    c: Composite,
+    flat_nodes: &mut Vec<Box<dyn Node>>,
+    flat_edges: &mut Vec<Edge>,
+) -> Result<ItemLowering, CompileError> {
+    let Composite { nodes, edges, input_roles, output } = c;
+    let item_count = nodes.len();
+
+    // the output port must name an in-range interior item (field range checked
+    // once the item's lowering is known)
+    if output.node >= item_count {
+        return Err(CompileError::OutputPortOutOfRange);
+    }
+
+    // recursively lower interior items, then rewrite interior edges through them
+    let interior = lower_items(nodes, flat_nodes, flat_edges)?;
+    for e in &edges {
+        for fe in rewrite_edge(e, &interior, flat_nodes)? {
+            flat_edges.push(fe);
+        }
+    }
+
+    // resolve the output port to a flat (node, field)
+    let out = match &interior[output.node] {
+        ItemLowering::Leaf { index } => {
+            if output.field >= flat_nodes[*index].schema().output.len() {
+                return Err(CompileError::OutputPortOutOfRange);
+            }
+            (*index, output.field)
+        }
+        ItemLowering::Composite { output: nested, .. } => {
+            // a nested composite exposes exactly one output field
+            if output.field != 0 {
+                return Err(CompileError::OutputPortOutOfRange);
+            }
+            *nested
+        }
+    };
+
+    // resolve each input role to flat targets (a target into a nested composite
+    // fans further) and kind-check every role
+    let mut roles: Vec<Vec<Target>> = Vec::with_capacity(input_roles.len());
+    for (r, role) in input_roles.iter().enumerate() {
+        let mut flat_targets: Vec<Target> = Vec::new();
+        for t in role {
+            flat_targets.extend(resolve_target(t, &interior)?);
+        }
+        if let Some((first, rest)) = flat_targets.split_first() {
+            let k0 = slot_kind(*first, flat_nodes)?;
+            for ft in rest {
+                if slot_kind(*ft, flat_nodes)? != k0 {
+                    return Err(CompileError::RoleKindMismatch { role: r });
+                }
+            }
+        }
+        roles.push(flat_targets);
+    }
+
+    Ok(ItemLowering::Composite { output: out, roles })
+}
+
+/// Rewrite one blueprint-level edge into flat edges. The `from` endpoint resolves
+/// to a single flat producer `(node, field)`; the `to` endpoint may fan out (a
+/// composite input role fans into several interior targets).
+fn rewrite_edge(
+    e: &Edge,
+    lowerings: &[ItemLowering],
+    flat_nodes: &[Box<dyn Node>],
+) -> Result<Vec<Edge>, CompileError> {
+    if e.from >= lowerings.len() {
+        return Err(CompileError::BadInteriorIndex);
+    }
+    let (from_node, from_field) = match &lowerings[e.from] {
+        ItemLowering::Leaf { index } => {
+            if e.from_field >= flat_nodes[*index].schema().output.len() {
+                return Err(CompileError::BadInteriorIndex);
+            }
+            (*index, e.from_field)
+        }
+        ItemLowering::Composite { output, .. } => {
+            // a composite exposes one output field; reading any other is malformed
+            if e.from_field != 0 {
+                return Err(CompileError::BadInteriorIndex);
+            }
+            *output
+        }
+    };
+    let targets = resolve_target(&Target { node: e.to, slot: e.slot }, lowerings)?;
+    Ok(targets
+        .into_iter()
+        .map(|t| Edge { from: from_node, to: t.node, slot: t.slot, from_field })
+        .collect())
+}
+
+/// Resolve a blueprint-level target `(node, slot)` into flat target(s). A target
+/// into a leaf is itself (remapped index); a target into a composite fans into
+/// that composite's input-role flat targets.
+fn resolve_target(t: &Target, lowerings: &[ItemLowering]) -> Result<Vec<Target>, CompileError> {
+    if t.node >= lowerings.len() {
+        return Err(CompileError::BadInteriorIndex);
+    }
+    match &lowerings[t.node] {
+        ItemLowering::Leaf { index } => Ok(vec![Target { node: *index, slot: t.slot }]),
+        ItemLowering::Composite { roles, .. } => {
+            let role = roles.get(t.slot).ok_or(CompileError::BadInteriorIndex)?;
+            Ok(role.clone())
+        }
+    }
+}
+
+/// The declared scalar kind of a flat node's input slot (for role kind-checking).
+fn slot_kind(t: Target, flat_nodes: &[Box<dyn Node>]) -> Result<ScalarKind, CompileError> {
+    flat_nodes[t.node]
+        .schema()
+        .inputs
+        .get(t.slot)
+        .map(|spec| spec.kind)
+        .ok_or(CompileError::BadInteriorIndex)
+}
+```
+
+- [ ] **Step 4: Add `CompileError` to the `lib.rs` re-export**
+
+In `crates/aura-engine/src/lib.rs`, extend the blueprint re-export line so it reads:
+
+```rust
+pub use blueprint::{Blueprint, BlueprintNode, CompileError, Composite, OutPort};
+```
+
+- [ ] **Step 5: Run the inliner tests to verify they pass**
+
+Run: `cargo test -p aura-engine blueprint::tests`
+Expected: PASS — `single_composite_inlines_with_offset_fan_and_output`,
+`nested_composite_inlines`, `bad_interior_index_rejected`,
+`role_kind_mismatch_rejected`, `output_port_out_of_range_rejected`,
+`bootstrap_error_is_wrapped`, and `composite_schema_derives_role_and_output_kinds`
+all pass; `0 failed`.
+
+---
+
+### Task 3: Headline acceptance — composite ≡ hand-wired, bit-for-bit (C1)
+
+**Files:**
+- Test: `crates/aura-engine/src/blueprint.rs` (inline `mod tests`)
+
+- [ ] **Step 1: Write the failing bit-identity demonstrator + fixtures**
+
+Append to the existing `#[cfg(test)] mod tests` in
+`crates/aura-engine/src/blueprint.rs`. Extend the test-module imports — change the
+existing `use` lines at the top of `mod tests` to also bring in the timestamp type,
+the std channel, and the `aura-std` nodes:
+
+```rust
+    use aura_core::{Ctx, FieldSpec, Firing, InputSpec, Scalar, Timestamp};
+    use aura_std::{Exposure, Recorder, SimBroker, Sma, Sub};
+    use std::sync::mpsc;
+```
+
+(The first line replaces the Task-1 `use aura_core::{Ctx, FieldSpec, Firing,
+InputSpec, Scalar};` line; the two new lines are added below it.)
+
+Then append the fixtures and the headline test:
+
+```rust
+    /// The built-in synthetic price stream (a local copy of the CLI sample's
+    /// stream): rises through t=4 then reverses, so the trace is non-degenerate.
+    fn synthetic_prices() -> Vec<(Timestamp, Scalar)> {
+        [
+            (1_i64, 1.0000_f64),
+            (2, 1.0010),
+            (3, 1.0030),
+            (4, 1.0060),
+            (5, 1.0040),
+            (6, 1.0010),
+            (7, 0.9990),
+        ]
+        .iter()
+        .map(|&(t, p)| (Timestamp(t), Scalar::F64(p)))
+        .collect()
+    }
+
+    /// Today's flat, hand-wired SMA-cross signal-quality harness (the
+    /// `sample_harness` wiring from `aura-cli`), with two recording sinks.
+    #[allow(clippy::type_complexity)]
+    fn hand_wired_sma_cross_harness() -> (
+        Harness,
+        mpsc::Receiver<(Timestamp, Vec<Scalar>)>,
+        mpsc::Receiver<(Timestamp, Vec<Scalar>)>,
+    ) {
+        let (tx_eq, rx_eq) = mpsc::channel();
+        let (tx_ex, rx_ex) = mpsc::channel();
+        let h = Harness::bootstrap(
+            vec![
+                Box::new(Sma::new(2)),
+                Box::new(Sma::new(4)),
+                Box::new(Sub::new()),
+                Box::new(Exposure::new(0.5)),
+                Box::new(SimBroker::new(0.0001)),
+                Box::new(Recorder::new(&[ScalarKind::F64], Firing::Any, tx_eq)),
+                Box::new(Recorder::new(&[ScalarKind::F64], Firing::Any, tx_ex)),
+            ],
+            vec![SourceSpec {
+                kind: ScalarKind::F64,
+                targets: vec![
+                    Target { node: 0, slot: 0 },
+                    Target { node: 1, slot: 0 },
+                    Target { node: 4, slot: 1 },
+                ],
+            }],
+            vec![
+                Edge { from: 0, to: 2, slot: 0, from_field: 0 },
+                Edge { from: 1, to: 2, slot: 1, from_field: 0 },
+                Edge { from: 2, to: 3, slot: 0, from_field: 0 },
+                Edge { from: 3, to: 4, slot: 0, from_field: 0 },
+                Edge { from: 4, to: 5, slot: 0, from_field: 0 },
+                Edge { from: 3, to: 6, slot: 0, from_field: 0 },
+            ],
+        )
+        .expect("valid hand-wired DAG");
+        (h, rx_eq, rx_ex)
+    }
+
+    /// The SMA-cross signal as a reusable composite: one input role (price), one
+    /// output (the fast-minus-slow spread). Interior wired with raw local indices.
+    fn sma_cross(fast: usize, slow: usize) -> Composite {
+        Composite::new(
+            vec![Sma::new(fast).into(), Sma::new(slow).into(), Sub::new().into()],
+            vec![
+                Edge { from: 0, to: 2, slot: 0, from_field: 0 },
+                Edge { from: 1, to: 2, slot: 1, from_field: 0 },
+            ],
+            vec![vec![Target { node: 0, slot: 0 }, Target { node: 1, slot: 0 }]],
+            OutPort { node: 2, field: 0 },
+        )
+    }
+
+    /// The same signal-quality harness authored as a composite blueprint.
+    #[allow(clippy::type_complexity)]
+    fn composite_sma_cross_harness() -> (
+        Blueprint,
+        mpsc::Receiver<(Timestamp, Vec<Scalar>)>,
+        mpsc::Receiver<(Timestamp, Vec<Scalar>)>,
+    ) {
+        let (tx_eq, rx_eq) = mpsc::channel();
+        let (tx_ex, rx_ex) = mpsc::channel();
+        let bp = Blueprint::new(
+            vec![
+                BlueprintNode::Composite(sma_cross(2, 4)),
+                Exposure::new(0.5).into(),
+                SimBroker::new(0.0001).into(),
+                Recorder::new(&[ScalarKind::F64], Firing::Any, tx_eq).into(),
+                Recorder::new(&[ScalarKind::F64], Firing::Any, tx_ex).into(),
+            ],
+            vec![SourceSpec {
+                kind: ScalarKind::F64,
+                targets: vec![
+                    Target { node: 0, slot: 0 }, // price -> sma_cross role 0
+                    Target { node: 2, slot: 1 }, // price -> SimBroker price slot
+                ],
+            }],
+            vec![
+                Edge { from: 0, to: 1, slot: 0, from_field: 0 }, // composite out -> Exposure
+                Edge { from: 1, to: 2, slot: 0, from_field: 0 }, // exposure -> broker slot 0
+                Edge { from: 2, to: 3, slot: 0, from_field: 0 }, // equity -> sink
+                Edge { from: 1, to: 4, slot: 0, from_field: 0 }, // exposure -> sink
+            ],
+        );
+        (bp, rx_eq, rx_ex)
+    }
+
+    #[test]
+    fn composite_sma_cross_runs_bit_identical_to_hand_wired() {
+        let prices = synthetic_prices();
+
+        // (a) today's flat, hand-wired graph
+        let (mut flat, flat_eq, flat_ex) = hand_wired_sma_cross_harness();
+        flat.run(vec![prices.clone()]);
+
+        // (b) the same graph authored as a composite blueprint, compiled
+        let (bp, comp_eq, comp_ex) = composite_sma_cross_harness();
+        let mut composed = bp.bootstrap().expect("composite blueprint compiles");
+        composed.run(vec![prices]);
+
+        let flat_eq_v = flat_eq.try_iter().collect::<Vec<_>>();
+        let flat_ex_v = flat_ex.try_iter().collect::<Vec<_>>();
+        let comp_eq_v = comp_eq.try_iter().collect::<Vec<_>>();
+        let comp_ex_v = comp_ex.try_iter().collect::<Vec<_>>();
+
+        // both recording sinks captured the same equity + exposure traces, bit-for-bit
+        assert_eq!(flat_eq_v, comp_eq_v, "equity traces differ");
+        assert_eq!(flat_ex_v, comp_ex_v, "exposure traces differ");
+        // and the trace is populated (non-degenerate), so the equality is meaningful
+        assert!(!comp_eq_v.is_empty(), "equity trace must be populated");
+        assert!(!comp_ex_v.is_empty(), "exposure trace must be populated");
+    }
+```
+
+- [ ] **Step 2: Run the headline test to verify it passes**
+
+Run: `cargo test -p aura-engine blueprint::tests::composite_sma_cross_runs_bit_identical_to_hand_wired`
+Expected: PASS (`test result: ok. 1 passed`). This is an acceptance test over the
+inliner built in Task 2 — the RED/GREEN boundary for the bit-identity property is
+between Task 2 (no `bootstrap()`) and Task 3. To confirm the assertion is
+load-bearing (not vacuously green on an empty trace), the test asserts the drained
+traces are non-empty; if it ever reports `0 passed; 0 filtered`, the test name in
+the filter is wrong — fall back to Step 3's unfiltered run.
+
+- [ ] **Step 3: Run the full engine test suite to verify everything passes**
+
+Run: `cargo test -p aura-engine`
+Expected: PASS — `composite_sma_cross_runs_bit_identical_to_hand_wired` passes
+alongside all Task-1/Task-2 tests and the pre-existing harness/report tests;
+`0 failed`.
+
+- [ ] **Step 4: Verify the workspace builds clean and lint is green**
+
+Run: `cargo clippy --workspace --all-targets -- -D warnings`
+Expected: PASS — no warnings. (Confirms no dead-code / unused-import regressions
+from the new module and that the demonstrator does not perturb the rest of the
+workspace.)