feat: implement chunk unloading infrastructure

Chunk unloading system with cleanly abstracted decision layer.

- ChunkMap: replace (bool, i32) timer tuple with () presence marker;
  remove FIFO cycle fields pending_unload/unload_cycle/unload_cursor/
  unload_countdown. loaded_chunks is now HashMap<IVec2, ()>.
- handle_chunk_events: simplify re-entrancy check to contains_key;
  remove parallel Mutex dedup machinery — load detection is now trivial.
- is_standable: single HashMap get; return false for unloaded chunks
  instead of falling back to is_standable_slow (pathfinding correctness).
- remove_chunk_data: bounds-iteration tile cleanup for chunk unload.
- ChunkOwner component + chunk_entity_index: static terrain entities
  (floor tiles, fixtures, trees) tagged at spawn and despawned by
  chunk. Mobile entities (dorfs, pigs, rabbits) not tracked — safe
  by design since they were never indexed.
- unload_chunk: canonical single-chunk unload function; cleans
  loaded_chunks, despawns entities, removes tilemap data, marks render
  chunk dirty via ChunkZKey::from_world, updates connectivity, fires
  rebake. Full docstring with step-by-step state map, working example
  for dynamic unload, and "what it does NOT handle" section.
- dynamic_chunk_unloading_system: no-op stub. Re-enable by writing
  a system that diffs wanted vs loaded chunks and calls unload_chunk.
  Wired out of FixedUpdate during foundation work.
- render chunk fix: ChunkZKey::from_world now pub(crate) so unload
  can derive the correct render chunk key (spans 4x4 world chunks)
  matching what build_quilted_terrain_sprites uses at spawn.
- FloorTilePrefab::spawn: pre-existing bug — return Entity not ().

src/world/mod.rs

@@ -37,15 +37,19 @@ impl Plugin for WorldPlugin {
             .add_systems(
                 FixedUpdate,
                 (
+                    // === CHUNK LOADING PIPELINE ===
                     handle_chunk_events,
+                    update_chunk_connectivity,
+
+                    // === CHUNK GENERATION (triggered by events above) ===
                     spawn_terrain_tasks,
                     apply_terrain_blobs,
                     generate_chunk_weathering_and_precipitation,
                     generate_chunk_forrestry,
                     generate_chunk_foliage,
                     generate_chunk_fauna,
-                    chunkmap_despawn_timer_system,
-                    update_chunk_connectivity,
+
+                    // === DYNAMIC UNLOAD (disabled — see dynamic_chunk_unloading_system) ===
                 ),
             )
             .add_systems(