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/simulation/rng.rs

@@ -2,6 +2,7 @@
 // Injectable ChaCha RNG resource for deterministic replay (D-030)
 // Ensures same seed produces same outcomes
 
+use crate::seed::splitmix64;
 use bevy_ecs::prelude::*;
 use rand::SeedableRng;
 use rand_chacha::ChaCha20Rng;
@@ -61,16 +62,6 @@ impl EntityRng {
     }
 }
 
-/// Splitmix64 mixing function — good avalanche properties for seed derivation.
-/// Ensures that similar inputs (e.g., consecutive StableIds) produce
-/// uncorrelated output seeds.
-fn splitmix64(mut x: u64) -> u64 {
-    x = x.wrapping_add(0x9e3779b97f4a7c15);
-    x = (x ^ (x >> 30)).wrapping_mul(0xbf58476d1ce4e5b9);
-    x = (x ^ (x >> 27)).wrapping_mul(0x94d049bb133111eb);
-    x ^ (x >> 31)
-}
-
 #[cfg(test)]
 mod tests {
     use super::*;