feat(simulation): FillChunk shell derivation — Void/Wall/FloorSlab/Roof from cached tags (T-987)

Implement the D-230 on-demand derive phase (data-production half). New
atlas/shell.rs derives a sparse building shell for one 64 m chunk from the
plan-phase BuildingPropertyTags: rectangle-containment (footprint ∩ chunk) ×
z-range (per-floor voxel bands from FloorExtent), in a quarter-ground z frame
(D-110). Walls on the footprint perimeter, FloorSlab on interior floor bases,
Roof above the top floor; interior air is Void and never stored.

- gen_queue.rs: GenWorkItem::FillChunk carries pre-resolved block_tags +
  block_pos + sub_chunk (run_work_item stays cache-free, mirroring
  GenerateSkeleton); GenCompletion::ChunkFilled carries Box<FilledChunk>; the
  arm calls shell::fill_chunk; pure build_fill_chunk_item(&QuarterWorldState,..)
  added.
- plugin.rs: ChunkFilled handler accepts the shell (trace-only — the Phase-5
  rendering consumer and the on-demand streaming dispatch trigger are deferred;
  do not build on legacy chunk_streaming.rs before Phase 5, gated by T-962).

Tests: determinism, sparsity-vs-dense-volume, wall/floor/roof correctness,
basement z-origin, sub-chunk clipping, full submit→drain→ChunkFilled round-trip.

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

server/src/atlas/mod.rs

@@ -24,6 +24,7 @@ pub mod plugin;
 pub mod region_profile;
 pub mod road_graph;
 pub mod scale;
+pub mod shell;
 pub mod skeleton_gen;
 pub mod source_resolver;
 pub mod subbiome;