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/Cargo.lock

@@ -1492,6 +1492,7 @@ dependencies = [
  "serde_json",
  "serde_norway",
  "sha2",
+ "smallvec",
  "sysinfo",
  "thiserror",
  "toml",
@@ -1551,6 +1552,9 @@ name = "smallvec"
 version = "1.15.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "67b1b7a3b5fe4f1376887184045fcf45c69e92af734b7aaddc05fb777b6fbd03"
+dependencies = [
+ "serde",
+]
 
 [[package]]
 name = "smol_str"