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/atlas/district_mix.rs

@@ -12,7 +12,7 @@
 //! **Determinism (D-010, D-194):** Integer weights throughout. No f32 in the
 //! district count computation. Seed-driven noise uses seeded RNG.
 
-use crate::atlas::rng::AtlasRng;
+use crate::seed::AtlasRng;
 use crate::simulation::generator::{DistrictType, PoliticalArchetype};
 
 // ---------------------------------------------------------------------------

server/src/atlas/mod.rs

@@ -12,7 +12,6 @@ pub mod features;
 pub mod gen_queue;
 pub mod heightmap;
 pub mod layer1;
-pub mod rng;
 pub mod skeleton_gen;
 pub mod subbiome;
 pub mod tile_condition;

server/src/atlas/rng.rs

@@ -1,57 +0,0 @@
-//! Seeded LCG for deterministic generation (D-010).
-//!
-//! Shared by all atlas generation modules that need seeded randomness.
-//! Uses Knuth's LCG parameters — integer-only arithmetic, no f32, D-010 compliant.
-//!
-//! Callers are responsible for any seed pre-mixing before calling `AtlasRng::new`.
-
-/// Seeded linear congruential generator (D-010).
-pub struct AtlasRng {
-    state: u64,
-}
-
-impl AtlasRng {
-    /// Create a new RNG from a pre-mixed seed.
-    ///
-    /// Callers must ensure the seed is non-degenerate (avoid passing 0 directly
-    /// if the seed could realistically be 0 — add a constant before calling).
-    pub fn new(seed: u64) -> Self {
-        Self { state: seed }
-    }
-
-    fn next_u64(&mut self) -> u64 {
-        self.state = self
-            .state
-            .wrapping_mul(6364136223846793005)
-            .wrapping_add(1442695040888963407);
-        self.state
-    }
-
-    /// Next pseudorandom `u32` (top 31 bits of the LCG state).
-    pub fn next_u32(&mut self) -> u32 {
-        (self.next_u64() >> 33) as u32
-    }
-}
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    #[test]
-    fn deterministic_sequence() {
-        let mut a = AtlasRng::new(42);
-        let mut b = AtlasRng::new(42);
-        for _ in 0..100 {
-            assert_eq!(a.next_u32(), b.next_u32());
-        }
-    }
-
-    #[test]
-    fn different_seeds_differ() {
-        let mut a = AtlasRng::new(1);
-        let mut b = AtlasRng::new(2);
-        let vals_a: Vec<u32> = (0..10).map(|_| a.next_u32()).collect();
-        let vals_b: Vec<u32> = (0..10).map(|_| b.next_u32()).collect();
-        assert_ne!(vals_a, vals_b);
-    }
-}

server/src/atlas/skeleton_gen.rs

@@ -20,7 +20,7 @@
 
 use crate::atlas::block_irregularity::block_irregularity;
 use crate::atlas::district_mix::{compute_district_mix, population_tier};
-use crate::atlas::rng::AtlasRng;
+use crate::seed::AtlasRng;
 use crate::simulation::generator::{
     BlockPlacement, BlockSkeleton, CityGenerationContext, ComplexityTier, DistrictId,
     DistrictLayoutMode, DistrictSkeleton, DistrictType, MultiBlockReservation, PoliticalArchetype,