Third walking-skeleton cycle: aura-engine's first real content — a Sim that
runs a wired DAG of nodes deterministically, cycle by cycle. First point where
authored nodes execute against data, not just under a hand-fed Ctx.
Engine shape (contract-forced): a flat Vec<NodeBox> + an index edge table,
topologically ordered (Kahn), owned &mut by the loop — the flat, monomorphized
sharpening of RustAst's Rc<RefCell<dyn Observer>> push graph (C1/C7). Each node
owns its input columns (cycle-0002 shape), so Ctx is unchanged and the per-cycle
path allocates nothing. Wiring is by forwarding: a producer's eval output is
pushed into its consumers' input columns; a None forwards nothing (the seed of
sample-and-hold).
Bootstrap generalizes C7's "type check paid once at wiring" to the whole
topology: size input columns from each node's schema, kind-check every edge,
reject directed cycles (BootstrapError). A worked 2-input Sub node lands in
aura-std so the loop is proven on a real fan-out+join DAG
(source -> {SMA(2), SMA(4)} -> Sub -> observe), with a determinism assertion (C1).
Deliberately deferred (recorded as decisions): freshness gating / sample-and-hold
(C5 -> cycle 0004, only testable once a second source at a different rate exists);
the Source trait + data-server ingestion + k-way merge (C3/C11); the ergonomic
builder API (C19); the real sink + run registry (C18/C22); cross-sim parallelism.
Grounding-check PASS: all assumptions about the 0001 substrate and 0002 node
contract ratified by green tests; no phantom methods.
refs walking-skeleton
Brummel
aaaf463102