Consolidate the node data structure so every node's signature (NodeSchema:
inputs/output/params) exists in the blueprint pre-build, declared once.
- Signature vs sizing split: NodeSchema becomes the static signature
(InputSpec -> PortSpec, lookback removed); the one param-dependent quantity
(input buffer lookback) moves to a build-time Node::lookbacks() query.
Node::schema() is removed.
- PrimitiveBuilder (ex-LeafFactory) carries the schema; the built node no
longer re-declares it -> closes the params-declared-twice drift.
- Blueprint collapses into Composite: Role gains source: Option<ScalarKind>;
the root is the fully-bound composite (no "main graph"); new error
UnboundRootRole.
- compile -> FlatGraph -> bootstrap: FlatGraph carries node + signature in
parallel; bootstrap sizes from lookbacks(), kind-checks from the signatures.
- Renames: BlueprintNode::Leaf -> Primitive, LeafFactory -> PrimitiveBuilder.
Behaviour-preserving (C1). Render is out of scope beyond compiling (next cycle).
refs #43#36
