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/atlas/mod.rs

@@ -8,11 +8,9 @@ pub mod block_irregularity;
 pub mod body_params_reader;
 pub mod body_world_state;
 pub mod cascade;
+pub mod chunk_context;
 pub mod city_context_reader;
 pub mod district_mix;
-// T-1026 foundation utility: covered by unit tests but not yet called from
-// production — its consumer is the T-1028 VoxelColumn pipeline (D-239 §4).
-#[allow(dead_code)]
 pub mod domain_warp;
 pub mod drainage;
 pub mod features;
@@ -26,5 +24,6 @@ pub mod skeleton_gen;
 pub mod source_resolver;
 pub mod subbiome;
 pub mod tile_condition;
+pub mod voxel;
 
 pub use plugin::GenerationPlugin;