refactor(simulation): remove superseded skeleton String stubs (T-984)

D-229 makes BuildingPropertyTag the typed step-3 output that replaces the era/
society_profile/zone_palette String stubs on the skeleton. The tag + its derivation
(zone_type_id, flavor_ref, era/ConstructionEra, FloorExtent) all landed under T-957/
T-1006; this removes the now-dead stub fields they superseded:

- BlockSkeleton.era_modifications: Vec<EraModification> (era is per-footprint on
  BuildingPropertyTag.era now; era_cause retained as the block-level cause)
- QuarterSkeleton.society_profile: SocietyProfileRef (culture → tag.flavor_ref)
- QuarterSkeleton.zone_palette: Vec<ZoneDefinition> (zoning → tag.zone_type_id;
  distinct from FloorZone.zone_palette: ZonePalette, the real D-101 palette, kept)
- the now-orphaned `EraModification` and `SocietyProfileRef` String aliases

Kept: chunk_layout (real ChunkLayout / D-234 street network), z_band_zones (still
uses the ZoneDefinition alias), boundaries (separate stub, not D-229 scope).

cargo check + clippy -D warnings --all-targets clean; full server lib suite
1325 passed / 0 failed.

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

server/src/atlas/skeleton_gen.rs

@@ -132,8 +132,6 @@ pub fn generate_quarter_skeleton(
         z_levels,
         social_sites: Vec::new(),
         access_points,
-        society_profile: String::new(),
-        zone_palette: Vec::new(),
         boundaries: String::new(),
         guarantee_audit: None,
     }
@@ -345,7 +343,6 @@ fn build_block_grid(
                     rotation_steps: 0,
                 },
                 hosted_sites: Vec::new(),
-                era_modifications: Vec::new(),
                 era_cause: None,
                 density_pct: density,
                 landmark: None,