feat(simulation): T-1119 QuarterFootprintLayer — server half (touch points 1-2) + populated wire fixtures

ZoningType gains PartialOrd/Ord (DistrictType T-994 mirror) for the
dominant_zoning declaration-order tie-break. layer_proxy:
QuarterFootprintEntry/QuarterFootprintLayer (BTreeMap keyed by
city_id; density_avg_pct floor-mean, dominant_* mode with
declaration-order tie-break, landmark/corridor counts) +
build_quarter_footprint_layer gated None on empty state.quarters;
sixth AtlasLayerResponse field wired into all construction sites;
growth-ceiling doc updated to say the whole-body budget is consumed
(what carries a seventh is T-1124's design, resolved in the same
branch's amendment). Placements whose derived Layer4Quarter id is not
yet in state.quarters are SKIPPED (skeleton gen is async Low-priority
— entries may be a proper subset; client decodes defensively). Stale
TODO(#957) removed (premise predates T-957 landing).

Tie-break bug caught by its own test: Iterator::max_by_key over a
BTreeMap returns the LAST max on ties — backwards from
declaration-order-first; replaced with an explicit forward walk that
replaces only on strictly-greater count.

gen_fixtures: populated RegionGridLayer (T-1118 replay coverage) +
QuarterFootprintLayer literals in the ready fixture; msgpack fixtures
regenerated via make fixtures. Note: landmark_count is inert on real
data today (BlockSkeleton.landmark hardcoded None until landmark
placement lands); fixtures use non-zero counts to exercise the logic.

5 new unit tests + populated round-trip; atlas:: 549 green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/src/atlas/plugin.rs

@@ -121,6 +121,7 @@ fn serve_atlas_requests(
                 road_graph: None,
                 settlements: None,
                 region_grid: None,
+                quarter_footprints: None,
             },
         };
         responses.0.push((conn_id, resp));
@@ -358,9 +359,7 @@ fn drain_generation_completions(
                         // 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 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.
+                        // the quarter's own stable id.
                         let _ = city_id; // used as dispatch key only; quarter_id is the map key
                         body_state
                             .quarters