fix(simulation): address PR #143 review (#1006)

- Add D-230 skeleton: DistrictSkeleton field to DistrictWorldState;
  cascade Default to DistrictSkeleton + contained enums/structs. Box the
  GenCompletion::SkeletonGenerated state to avoid large_enum_variant.
- Derive PartialEq on FloorExtent/FloorHeightProfile/DistrictWorldState/
  CityGenerationContext (+ minimal cascade) for downstream assert_eq tests.
- Add 4 unit tests for floor_at_voxel_z / voxel_range_for_floor (uniform,
  basement, variable heights, boundary) — the Q-104 deliverable.
- Drop unused smallvec direct dep (stays transitive via bevy_ecs).
- Key districts insert by skeleton.district_id, sharpen TODO(#957).

clippy --all-targets -D warnings clean; 1263 lib tests pass; fmt clean.

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

server/src/atlas/gen_queue.rs

@@ -103,8 +103,9 @@ pub enum GenCompletion {
         /// The body this skeleton belongs to — used to route state into
         /// `BodyWorldState.districts` (D-230).
         body_id: String,
-        /// District-level world state (block tags) produced by the plan phase (D-230).
-        state: DistrictWorldState,
+        /// District-level world state (skeleton + block tags) produced by the plan phase (D-230).
+        /// Boxed to keep `GenCompletion` variant sizes balanced (D-230 skeleton is ~2.7 KB).
+        state: Box<DistrictWorldState>,
     },
     ChunkFilled {
         district_id: u64,
@@ -344,7 +345,7 @@ fn run_work_item(item: &GenWorkItem) -> GenCompletion {
             GenCompletion::SkeletonGenerated {
                 city_id: *city_id,
                 body_id: String::new(),
-                state: DistrictWorldState::default(),
+                state: Box::new(DistrictWorldState::default()),
             }
         }
         GenWorkItem::FillChunk {