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/gen_queue.rs

@@ -111,10 +111,7 @@ pub enum GenCompletion {
         block_pos: (u32, u32),
     },
     /// Work item failed — body_id or city_id for logging.
-    Failed {
-        item: GenWorkItem,
-        reason: String,
-    },
+    Failed { item: GenWorkItem, reason: String },
 }
 
 // ---------------------------------------------------------------------------