feat: data-oriented chunks optimization with tile registry

- Phase 1: Bit-packed standability (ChunkData with bitsets)
- Phase 2: Reactive connectivity (dirty chunks)
- Phase 3: Async terrain baking (AsyncComputeTaskPool)
- Pathfinding weight system (rock=50 preferred, bedrock=150 avoided)
- Movement speed affected by tile weight
- External tiles.toml for hot-reloadable tile definitions
- TileRegistry singleton for async-safe tile lookups
- Fixed world_to_chunk to use CHUNK_SIZE_TILE (128) not CHUNK_SIZE (8)
- Fixed infinite spawner with Local<bool> state guards
- Fixed spawn coordinate grid alignment

Note: Zigzag pathfinding bug introduced - needs investigation

src/world/tiles/tilemap.rs

@@ -269,4 +269,18 @@ impl TileMap {
 
         (can_stand_in_floor || can_stand_in_fixture) && (can_stand_on_floor || can_stand_on_fixture)
     }
+
+    /// Get A* pathfinding weight for a tile position.
+    /// Returns 100 (default) if tile not found. Lower is better.
+    pub fn get_astar_weight(&self, world_pos: IVec3) -> u8 {
+        let chunk_pos = world_to_chunk(world_pos);
+        if let Some(chunk) = self.chunks.get(&chunk_pos) {
+            let (local_x, local_y, z) = ChunkData::world_to_local(world_pos);
+            return chunk.get_astar_weight(local_x, local_y, z);
+        }
+        self.floor_tiles
+            .get(&world_pos)
+            .map(|t| t.astar_weight)
+            .unwrap_or(100)
+    }
 }