feat(simulation): foundation shared types + struct plumbing (#1006)

Define the type-definition layer the Phase-4 fill-seam tickets depend on
(D-229/D-230/D-231/D-232/D-233), compiling with stubs/defaults; behavior
logic lands in #982-985/#998.

- New types in generator.rs: BuildingPropertyTag, FloorExtent +
  FloorHeightProfile (floor_at_voxel_z/voxel_range_for_floor, resolves
  Q-104), BuildingEntryClass, ConstructionEra, ZoneTypeId, MorphologyZone,
  BulkClass(5), ProductionUbiquity, DoorSpec, InteriorDescriptor,
  DistrictWorldState.
- Rename spatial AccessTier -> ZoneAccessTier to free the name for the new
  per-building BuildingEntryClass.
- CityGenerationContext: +morphology_zone, +trait_selection,
  +dominant_bulk_class, +dominant_production_ubiquity.
- BodyWorldState: +districts (DistrictWorldState w/ block_tags).
- GenCompletion::SkeletonGenerated carries body_id + DistrictWorldState;
  plugin handler inserts into BodyWorldState.districts.
- Add smallvec as a direct dep (DoorSpec list stays Vec for now, TODO).

cargo check --all-targets / clippy clean; 1259 lib tests pass.

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

server/src/atlas/plugin.rs

@@ -80,8 +80,26 @@ fn drain_generation_completions(
             GenCompletion::Failed { item, reason } => {
                 tracing::warn!(?item, %reason, "background generation work item failed");
             }
-            // Produced once the later layers land (#957 / #959); no consumer yet.
-            GenCompletion::SkeletonGenerated { .. } | GenCompletion::ChunkFilled { .. } => {}
+            // Insert district world state into the matching body's cache entry (D-230).
+            GenCompletion::SkeletonGenerated {
+                city_id,
+                body_id,
+                state,
+            } => {
+                if !body_id.is_empty() {
+                    if let Some(body_state) = cache.peek_mut(&body_id) {
+                        body_state.districts.insert(city_id, state);
+                    } else {
+                        tracing::warn!(
+                            city_id,
+                            body_id,
+                            "SkeletonGenerated: body not in cache — district state dropped"
+                        );
+                    }
+                }
+                // body_id empty = stub result from GenerateSkeleton stub; silently ignore.
+            }
+            GenCompletion::ChunkFilled { .. } => {}
         }
     }
 }