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/Cargo.lock

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