Wrap TileMap HashMaps in Arc for async pathfinding access

- Add Arc<AHashMap> wrapper around floor_tiles, fixture_tiles, item_tiles
- Copy-on-write semantics: Arc::make_mut clones only if other Arcs exist
- Add insert_floor, insert_fixture, insert_item, remove_item methods
- Add get_floor_mut, get_fixture_mut for visibility updates
- Update all mutation sites to use new TileMap methods
- Enables cheap Arc::clone for async pathfinding workers
- Single-threaded pathfinding: no clone, direct access
- Multi-threaded pathfinding: clone Arc, read without locks

src/world/tiles/tilemap.rs

@@ -1,5 +1,6 @@
 use ahash::AHashMap;
 use bevy::prelude::*;
+use std::sync::Arc;
 
 /// Packed floor tile data for efficient storage. ~35 bytes vs 76 bytes tuple.
 #[derive(Clone, Copy, Debug)]
@@ -170,12 +171,13 @@ impl FixtureTileData {
     }
 }
 
-/// Tile map with fast AHashMap for pathfinding lookups.
-#[derive(Resource, Default)]
+/// Tile map with Arc-wrapped HashMaps for async pathfinding access.
+/// Uses copy-on-write: Arc::make_mut clones only if other Arcs exist.
+#[derive(Resource, Clone, Default)]
 pub struct TileMap {
-    pub floor_tiles: AHashMap<IVec3, FloorTileData>,
-    pub fixture_tiles: AHashMap<IVec3, FixtureTileData>,
-    pub item_tiles: AHashMap<IVec3, Vec<u32>>,
+    pub floor_tiles: Arc<AHashMap<IVec3, FloorTileData>>,
+    pub fixture_tiles: Arc<AHashMap<IVec3, FixtureTileData>>,
+    pub item_tiles: Arc<AHashMap<IVec3, Vec<u32>>>,
 }
 
 impl TileMap {
@@ -202,4 +204,37 @@ impl TileMap {
     pub fn has_fixture(&self, pos: &IVec3) -> bool {
         self.fixture_tiles.contains_key(pos)
     }
+
+    #[inline]
+    pub fn insert_floor(&mut self, pos: IVec3, tile: FloorTileData) {
+        Arc::make_mut(&mut self.floor_tiles).insert(pos, tile);
+    }
+
+    #[inline]
+    pub fn insert_fixture(&mut self, pos: IVec3, tile: FixtureTileData) {
+        Arc::make_mut(&mut self.fixture_tiles).insert(pos, tile);
+    }
+
+    #[inline]
+    pub fn insert_item(&mut self, pos: IVec3, entity_id: u32) {
+        Arc::make_mut(&mut self.item_tiles)
+            .entry(pos)
+            .or_default()
+            .push(entity_id);
+    }
+
+    #[inline]
+    pub fn remove_item(&mut self, pos: &IVec3) -> Option<Vec<u32>> {
+        Arc::make_mut(&mut self.item_tiles).remove(pos)
+    }
+
+    #[inline]
+    pub fn get_floor_mut(&mut self, pos: &IVec3) -> Option<&mut FloorTileData> {
+        Arc::make_mut(&mut self.floor_tiles).get_mut(pos)
+    }
+
+    #[inline]
+    pub fn get_fixture_mut(&mut self, pos: &IVec3) -> Option<&mut FixtureTileData> {
+        Arc::make_mut(&mut self.fixture_tiles).get_mut(pos)
+    }
 }