A seeded source whose stream is fully determined by a u64 seed, making
RunManifest.seed a live captured input instead of the dead 0 it was. This is
C12's reconciliation of stochastic runs with C1: a run is bit-identical for a
fixed seed (the seed is a captured input, not hidden nondeterminism), and a
different seed gives a different-but-reproducible run. Precondition for the
Monte-Carlo family (#68) and random param-sweep (#52).
Engine (aura-engine):
- SplitMix64: a ~10-line private, dependency-free PRNG. Chosen over a rand-family
crate for guaranteed bit-stability across toolchains/crate versions — the whole
value of seed-as-input is reproducibility, and a crate whose algorithm is not
version-stable would silently break a recorded seed on a dep bump. No new direct
dep.
- SyntheticSpec + source(seed) -> impl Source: the Fn(u64) -> impl Source contract
(Fork B — seed at the data-generation edge, outside the engine graph). Returns a
Source, never a Vec, so the MC family can re-seed N times without materializing
N streams. First cut collects to a VecSource internally; the signature does not
name Vec, so a lazy generator is a drop-in replacement. Re-exported from lib.rs
beside VecSource.
CLI (aura-cli):
- sim_optimal_manifest gains a seed: u64 param recorded into the manifest (the
single Fork B capture site). All four existing seed-free callers (run_sample,
run_sample_real, the sweep grid-report, run_macd) pass 0 unchanged — their
determinism tests stay green.
- A test-only run_sample_seeded + SeededTrace exercise a live seed and expose the
drained sink trace, so the three e2e tests assert bit-identity at the trace
level (strictly stronger than the folded 3-field metrics).
Tests (RED-first): 2 engine producer tests (same-seed identical / different-seed
differs) + 3 cli e2e tests (bit-identical trace, different-seed metrics differ,
seed recorded in manifest). Full workspace green; clippy -D warnings clean.
Errata vs plan 0042 (two compile-forced deviations, neither alters the spec
contract):
- source returns `impl Source + use<>`: under edition 2024 a bare `impl Source`
captures the &self lifetime, which blocks boxing as Box<dyn Source + 'static>
for run() (E0597). The source owns its data, so use<> (captures nothing) is
correct.
- SyntheticSpec is imported inside `#[cfg(test)] mod tests` in main.rs: it is
used only by the seeded test vehicle, so a top-level import is unused in the
non-test build under clippy -D warnings.
closes#66
Brummel
c9f0e438e1