feat(simulation): SeedChain seed-derivation primitive in server/src/seed.rs (#952, D-224)

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>

server/src/lib.rs

@@ -8,6 +8,7 @@ pub mod cause_chain;
 pub mod knowledge;
 pub mod npc;
 pub mod perception;
+pub mod seed;
 pub mod settings;
 pub mod simulation;
 pub mod storyteller;