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/skeleton_gen.rs

@@ -411,7 +411,8 @@ fn derive_reservations(
 mod tests {
     use super::*;
     use crate::simulation::generator::{
-        CityGenerationContext, FoundingOrientation, PoliticalArchetype, SettingType, WorldTier,
+        BulkClass, CityGenerationContext, FoundingOrientation, MorphologyZone, PoliticalArchetype,
+        ProductionUbiquity, SettingType, WorldTier,
     };
 
     fn make_context(archetype: PoliticalArchetype, world_tier: WorldTier) -> CityGenerationContext {
@@ -424,6 +425,11 @@ mod tests {
             footprint_radius_km: 10.0,
             founding_orientation: FoundingOrientation::Cardinal,
             world_tier,
+            // D-229/D-232/D-233 additions — sensible stubs for existing tests.
+            morphology_zone: MorphologyZone::AlluvialPlain,
+            trait_selection: Vec::new(),
+            dominant_bulk_class: BulkClass::NonPhysical,
+            dominant_production_ubiquity: ProductionUbiquity::Common,
         }
     }