feat(simulation): quarter footprint subdivision + block-tag assembly (#957)

Build the per-quarter building geometry + tags (absorbs #976's footprint
subdivision into #957 — one coherent walkable-quarter deliverable):

- subdivide_block_footprints: deterministic BSP of each 128x128-tile block into
  variable axis-aligned building plots (D-229 fast-path). Lot size + setback
  scale with D-220 density (Frontier -> few big lots/wide gaps; Compressed ->
  many small lots/shared walls); the D-233 BulkClass roofed-coverage fraction
  decides building-vs-interstitial. morphology is wired in for the D-234
  waterfront/street layer (needs Layer-1 terrain water-adjacency threaded up).
- assign_all_block_tags: subdivides + tags every non-reserved block's footprints
  via the #957 derivation helpers, populating QuarterWorldState.block_tags in the
  GenerateSkeleton plan-phase (D-230). Reserved blocks get no standard fill.
- Remove the write-only BlockSkeleton.era String stub - construction era now
  lives per-footprint on BuildingPropertyTag.era (typed ConstructionEra).

All integer-deterministic (D-010). 5 new tests. Doors stay Vec::new() (#979).

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

server/src/simulation/generator.rs

@@ -1052,7 +1052,8 @@ pub struct BlockSkeleton {
     pub reservation: Option<ReservationId>,
     pub chunk_layout: ChunkLayout,
     pub hosted_sites: Vec<SocialSiteId>,
-    pub era: Era,
+    // Per-block `era: Era` (String stub) removed in #957 — construction era now
+    // lives per-footprint on `BuildingPropertyTag.era` (typed `ConstructionEra`).
     pub era_modifications: Vec<EraModification>,
     pub era_cause: Option<EraCause>,
     /// Build density percentage (0 = open/empty, 100 = fully built-up).