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/tile_condition.rs

@@ -23,6 +23,8 @@
 //!
 //! Threshold values are authored constants (D-217): 0.63, 0.43, 0.23.
 
+use serde::{Deserialize, Serialize};
+
 use crate::simulation::generator::EraCause;
 
 // ---------------------------------------------------------------------------
@@ -30,7 +32,7 @@ use crate::simulation::generator::EraCause;
 // ---------------------------------------------------------------------------
 
 /// Visual condition band for a tile, derived from prosperity_score (D-217).
-#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
+#[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
 pub enum TileCondition {
     /// prosperity_score > 0.63. Clean, undamaged, well-maintained.
     Intact,