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/simulation/generator.rs

@@ -231,7 +231,13 @@ pub enum DistrictLayoutMode {
 }
 
 /// Zoning classification for a block or floor zone.
-#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, Eq, Default)]
+///
+/// `Ord`/`PartialOrd` (T-1119, mirroring `DistrictType`'s T-994 precedent above):
+/// lets the quarter-footprint aggregate (`layer_proxy::build_quarter_footprint_layer`)
+/// resolve a `dominant_zoning` mode-tie by lowest declaration-order variant.
+/// Declaration order is not a stability-pinned wire format on this enum (unlike
+/// `MorphologyZone` or `SeedDomain`), so this is a safe additive derive.
+#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Default)]
 pub enum ZoningType {
     Commercial,
     Residential,