A seeded source whose stream is fully determined by a u64 seed, wired so the
seed reaches RunManifest.seed at the manifest-construction site (Fork B: seed at
the data-generation edge, outside the engine graph). Contract is a producer
Fn(u64) -> impl Source, never seed -> Vec, so the Monte-Carlo family can re-seed
N times without materializing N streams. Deterministic dependency-free PRNG
(SplitMix64) for C1 bit-stability. Precondition for #68 (Monte-Carlo) and #52
(random sweep).
Auto-signed under /boss spec auto-sign: objective gates green (precondition,
self-review, grounding-check PASS) and a unanimous five-lens spec-skeptic panel
(criterion, grounding, scope-fork, ambiguity, plan-readiness all SOUND) after one
editorial-repair round.
refs #66
Brummel
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