New top-level seed module that owns every deterministic-RNG primitive:
- splitmix64 — the one canonical mixer (was duplicated as a private fn in
simulation/rng.rs; EntityRng now imports the shared one, no behavior change)
- AtlasRng — moved here from atlas/rng.rs (it is a generation-RNG primitive,
not atlas-specific); atlas now depends on seed, not the reverse
- SeedDomain — append-only domain tags (Body/Layer1Topography/Layer3Settlement/
Layer4Quarter/Block/Npc) for collision-proof per-domain seed separation
- SeedChain — root(world_seed) → derive(domain, id) → atlas_rng()/seed(), per
the D-224 formula splitmix64(self ^ splitmix64(domain)) ^ splitmix64(id)
Structural move only — SeedChain is not yet threaded through the cascade
callers (skeleton_gen still uses ad-hoc wrapping_add pre-mixing); that is the
next step. Unit tests cover the splitmix64 known-vector, avalanche,
determinism, domain/id separation, and chain composition.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
jpmschweitzerandClaude Opus 4.7
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