fix(simulation): review follow-ups on foundation types (#1006)

- Derive MorphologyZone Default (#[default] AlluvialPlain), drop manual
  impl — clears the clippy --all-targets warning (pre-push gate).
- Fix floor_at_voxel_z doc (described the inverse function).
- Document floor_at_voxel_z / voxel_range_for_floor voxel-z as
  building-relative (base_floor = z:0) for #982-985 callers.
- TODO(#957): districts keyed by city_id is a stub; use district_id.

clippy --all-targets clean; 1259 lib tests pass.

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

server/src/atlas/plugin.rs

@@ -88,6 +88,8 @@ fn drain_generation_completions(
             } => {
                 if !body_id.is_empty() {
                     if let Some(body_state) = cache.peek_mut(&body_id) {
+                        // TODO(#957): keying by city_id is a stub — a city has multiple districts;
+                        // use DistrictSkeleton.district_id as the DistrictId key when real skeleton gen lands.
                         body_state.districts.insert(city_id, state);
                     } else {
                         tracing::warn!(