audit: cycle 0042 — drift-clean (seed-as-input)

Architect drift review over 682e459..HEAD (the seed-as-input cycle plus the
prior unaudited `aura run --real` work). Cycle is sound: Fork B respected (seed
forks at the data-generation edge into the source and the manifest; the engine
loop sees only a Source — C12/C1/C3 intact), the contract names Source never Vec
(precondition for #68/#52 delivered as specced), and the four seed-free callers
thread 0 in lockstep.

One medium doc-lag item found and fixed here: aura-engine/src/lib.rs module-doc
listed the seed axis under "still to come"; seed-as-input shipped this cycle and
RunManifest.seed is now live, so the headline doc now records SyntheticSpec as a
delivered seeded Source producer (params + orchestration families remain future).

One low carry-on note (not drift): run_sample_seeded exists only under
#[cfg(test)] — intended this cycle (no --seed CLI flag per spec non-goals); the
only non-test seed value remains 0. A scaling marker for #68, not debt.

Regression gate (test suite — no dedicated regression scripts in project facts):
cargo test --workspace green; clippy --workspace --all-targets -D warnings clean;
cargo doc -p aura-engine clean (the new [`SyntheticSpec`] intra-doc link resolves).

crates/aura-engine/src/lib.rs

@@ -26,7 +26,10 @@
 //! position-event output and downstream broker nodes (C10), and the param
 //! injection + orchestration axes of the atomic sim unit
 //! (`(topology + params + data-window + seed)`, swept by optimize / walk-forward
-//! / Monte-Carlo) — its `-> metrics` reduction now ships via [`summarize`].
+//! / Monte-Carlo). The **seed** axis now ships: [`SyntheticSpec`] is a seeded
+//! `Source` producer (`Fn(u64) -> impl Source`, C12 seed-as-input) whose stream
+//! is fully seed-determined, making [`RunManifest`]'s seed a live captured input;
+//! the atomic unit's `-> metrics` reduction ships via [`summarize`].
 //!
 //! Visualization is never here: it is a downstream consumer node on the streams.