fix(simulation): address PR #161 review (VoxelColumn skeleton)

Hoshe REQUEST_CHANGES + Tyre APPROVE:

- Cache key omitted world_seed → a cache reused across worlds could return
  another seed's column. CacheKey is now (world_seed, body_id_hash, voxel_pos).
  (Hoshe #1)
- Sub-chunk voxel seed reused SeedDomain::ChunkContext (tier-collision risk vs
  the region meander seed). Add SeedDomain::Voxel = 9 (append-only, pin test
  updated) and use it. (Hoshe #2, Tyre #1)
- get_or_derive forced callers to pre-compute body_id_hash while fnv1a_64 is
  pub(crate). Drop the param; compute the hash internally. (Tyre #2)
- Fix two inaccurate comments (micro-relief range [-4,+3] not ±2; scatter mask
  is 4-bit [0,15] not bits [4:6]). (Hoshe #3/#4)
- Strengthen the LRU eviction test: add a test-only contains_voxel() helper and
  assert WHICH entry was evicted (refreshed pos survives, LRU pos evicted). (Hoshe #5)
- LRU O(capacity)-scan perf flagged on T-1031 (Tyre #3, deferred to the budget harness).

cargo test passes, clippy -D warnings clean, fmt clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/src/seed.rs

@@ -100,6 +100,11 @@ pub enum SeedDomain {
     /// ChunkContext derivation (64 m carrier, D-239 §1, T-1028).
     /// Keyed by region-scale position id (see `atlas::chunk_context::pos_to_id`).
     ChunkContext = 8,
+    /// Per-voxel sub-chunk derivation (1 m, D-239 §10, T-1028). Keyed by the
+    /// post-warp integer voxel-position id. Distinct from `ChunkContext` so the
+    /// voxel stream can never collide with the region-scale meander seed (D-224
+    /// domain separation).
+    Voxel = 9,
 }
 
 /// A position in the deterministic seed tree (D-224).
@@ -257,6 +262,7 @@ mod tests {
         assert_eq!(SeedDomain::Npc as u64, 6);
         assert_eq!(SeedDomain::DomainWarp as u64, 7);
         assert_eq!(SeedDomain::ChunkContext as u64, 8);
+        assert_eq!(SeedDomain::Voxel as u64, 9);
     }
 
     #[test]