refactor(simulation): rename DistrictSkeleton→QuarterSkeleton per D-222 (#950)

D-222 renamed the 512m generation cell from District to Quarter (District
is now a new 2048m tier above it). Align the generation skeleton code to
the canonical vocabulary. Pure naming — no behavior change; all 1296 lib
tests + integration tests pass unchanged.

Renamed (spatial-cell identifiers):
- DistrictSkeleton → QuarterSkeleton, DistrictWorldState → QuarterWorldState
- DistrictId → QuarterId, DistrictContext/DistrictBoundaries → Quarter*
- field district_id → quarter_id, district_type → quarter_type
- BodyWorldState.districts map → .quarters
- generate_skeleton → generate_quarter_skeleton

Deliberately left as-is (these name functional ZONING, not the spatial
tier — orthogonal to D-222): DistrictType, DistrictLayoutMode, the
district_mix module (DistrictMix/compute_district_mix), and the
GenWorkItem::GenerateSkeleton / GenCompletion::SkeletonGenerated variants.

Also aligned the perception "sim tile" → "subtile" vocabulary (D-222:
Subtile = 0.5m) in decisions/perception.md and the generation-cascade code
comments. Historical D-066/D-094/D-201/D-220 decision bodies keep their
existing D-222 amendment notes (not rewritten in place); the public
max_offset_sim_tiles fn name is unchanged.

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

server/src/atlas/plugin.rs

@@ -93,16 +93,16 @@ fn drain_generation_completions(
             } => {
                 if !body_id.is_empty() {
                     if let Some(body_state) = cache.peek_mut(&body_id) {
-                        // Key by state.skeleton.district_id (D-194/D-230): a city has many
-                        // districts, each with its own DistrictId. `city_id` is only the
+                        // Key by state.skeleton.quarter_id (D-194/D-230): a city has many
+                        // quarters, each with its own QuarterId. `city_id` is only the
                         // dispatch key used in the work item — the canonical insert key is
-                        // the district's own stable id. TODO(#957): the stub GenerateSkeleton
-                        // returns a default skeleton with district_id=0; real gen (#957) will
+                        // the quarter's own stable id. TODO(#957): the stub GenerateSkeleton
+                        // returns a default skeleton with quarter_id=0; real gen (#957) will
                         // populate it from CityGenerationContext.
-                        let _ = city_id; // used as dispatch key only; district_id is the map key
+                        let _ = city_id; // used as dispatch key only; quarter_id is the map key
                         body_state
-                            .districts
-                            .insert(state.skeleton.district_id, *state);
+                            .quarters
+                            .insert(state.skeleton.quarter_id, *state);
                     } else {
                         tracing::warn!(
                             city_id,