perf: eliminate HashMap lookups on z-scroll with fast recolor + targeted boundary dirty

Three optimizations for z-scroll performance:

Option 1 — Fast in-place recolor (no HashMap lookups):
- TilemapChunkStates stores solid_bits + underground_bits per chunk
- recolor_chunk_for_depth iterates flat Vec<Option<TileData>> — pure cache-friendly sequential reads
- tile_fade_color computes color from tileset_idx + z_diff without any HashMap access
- vs repopulate_chunk_tiles: 32K Vec reads vs 32K HashMap lookups per frame

Option 2 — Targeted boundary dirty (from 9×chunks to 2×chunks):
- on_camera_z_changed now only marks the 4 boundary z-levels:
  old_entered, old_exited, new_entered, new_exited
- Registry.z_change_keys tracks z-changed keys separately from occlusion dirty_keys
- populate_tilemap_chunk_data uses fast recolor for z_change_keys,
  full repopulate for dirty_keys (occlusion — rare)

Option 3 — Separate budgets:
- z_change_keys processed with MAX_POPULATE_PER_FRAME budget (fast recolor)
- dirty_keys processed with MAX_POPULATE_PER_FRAME budget (full repopulate)
- Both pipelines tracked separately in benchmark

Also:
- populate_chunk_tiles now returns underground_bits for state storage
- compute_solid_bits helper extracts solid tiles from TileData Vec
- spawn_tilemap_chunks computes and stores both bitmasks on spawn
- despawn_tilemap_chunks clears all state for despawned chunks

config.toml

@@ -1,4 +1,4 @@
-initial_chunk_radius = 5
+initial_chunk_radius = 15
 
 [spawn_counts]
 dorfs = 5