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/plugin.rs

@@ -88,9 +88,16 @@ fn drain_generation_completions(
             } => {
                 if !body_id.is_empty() {
                     if let Some(body_state) = cache.peek_mut(&body_id) {
-                        // TODO(#957): keying by city_id is a stub — a city has multiple districts;
-                        // use DistrictSkeleton.district_id as the DistrictId key when real skeleton gen lands.
-                        body_state.districts.insert(city_id, state);
+                        // Key by state.skeleton.district_id (D-194/D-230): a city has many
+                        // districts, each with its own DistrictId. `city_id` is only the
+                        // dispatch key used in the work item — the canonical insert key is
+                        // the district's own stable id. TODO(#957): the stub GenerateSkeleton
+                        // returns a default skeleton with district_id=0; real gen (#957) will
+                        // populate it from CityGenerationContext.
+                        let _ = city_id; // used as dispatch key only; district_id is the map key
+                        body_state
+                            .districts
+                            .insert(state.skeleton.district_id, *state);
                     } else {
                         tracing::warn!(
                             city_id,