feat(simulation): VoxelColumn walking skeleton (T-1028)

D-239 §1/§7/§10 — the lower derivation chain end-to-end with the AlluvialPlain
fallback only (specialized families are T-1029).

- chunk_context.rs: ChunkContext (64m) — basin_direction + meander phase/wavelength
  + active-channel params. NO per-tile flow_direction[64x64] (D8 ~152m coarser than
  a chunk, D-239 §10); features >64m seed from Region-or-higher. SeedDomain::ChunkContext=8.
- voxel.rs: VoxelColumn (1m) composing the D-228 tile axes (TerrainMaterial /
  FloorMaterial=None / Vegetation / Water / elevation_m). Derive-on-demand + VoxelCache
  (LRU BTreeMap, never persisted per D-227). Domain warp (T-1026) wired through:
  f64 warp -> as-i32 truncation to the voxel address; all material/morphology gates
  integer (D-010 — the meander wavelength f64 is truncated before the Water decision).
- AlluvialPlain generator: Soil/Wetland terrain, climate vegetation, integer-meander
  channel (Shallow/Deep) from ChunkContext. The other 7 families are dispatch stubs
  (canonical terrain material, fall back to AlluvialPlain geometry) — no panic.

29 new tests incl end-to-end determinism (same seed/pos -> identical column),
cache hit/miss/eviction/re-derivation, warp-applied. cargo test 1563 pass,
clippy -D warnings clean, fmt clean.

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

server/src/seed.rs

@@ -97,6 +97,9 @@ pub enum SeedDomain {
     /// Anti-squaring domain warp (D-239 §4, T-1026).
     /// Keyed by per-tile position id (see `atlas::domain_warp::pos_to_id`).
     DomainWarp = 7,
+    /// 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,
 }
 
 /// A position in the deterministic seed tree (D-224).
@@ -253,6 +256,7 @@ mod tests {
         assert_eq!(SeedDomain::Block as u64, 5);
         assert_eq!(SeedDomain::Npc as u64, 6);
         assert_eq!(SeedDomain::DomainWarp as u64, 7);
+        assert_eq!(SeedDomain::ChunkContext as u64, 8);
     }
 
     #[test]