Fix performance regressions: remove TIER0, consolidate scratchpads, remove Arc trap

Key fixes based on benchmark analysis:
1. Remove TIER0 Vec-based pathfinding - O(N) linear scan was slower than FxHashMap for typical path lengths
2. Consolidate scratchpads into single AStarScratchpad struct - eliminates nested RefCell borrow overhead
3. Remove Arc wrapper from TileMap - eliminated Copy-on-Write trap causing 63ms stutters
4. Replace AHashMap with FxHashMap - FxHash is faster for small integer keys like IVec3
5. Simplify tier logic - single pathfinding function with scratchpad reuse

Benchmark analysis showed:
- Original: 1.95 µs/node, P99 1.1ms, Max 5ms
- TIER0/TIER1 regression: 2.89 µs/node (+48%), P99 5ms (+348%)
- Root causes: Vec linear scan in TIER0, nested RefCell borrows, Arc::make_mut CoW

This should restore and improve performance by using simple FxHashMap scratchpad for all paths.

src/world/tiles/tilemap.rs

@@ -1,6 +1,5 @@
-use ahash::AHashMap;
 use bevy::prelude::*;
-use std::sync::Arc;
+use rustc_hash::FxHashMap;
 
 /// Packed floor tile data for efficient storage. ~35 bytes vs 76 bytes tuple.
 #[derive(Clone, Copy, Debug)]
@@ -89,28 +88,6 @@ impl FloorTileData {
             self.flags &= !0b100;
         }
     }
-
-    pub fn from_tuple(tuple: (i32, bool, bool, bool, i32, [u32; 8])) -> Self {
-        Self::new(
-            tuple.0 as u8,
-            tuple.1,
-            tuple.2,
-            tuple.3,
-            tuple.4 as u8,
-            tuple.5,
-        )
-    }
-
-    pub fn to_tuple(&self) -> (i32, bool, bool, bool, i32, [u32; 8]) {
-        (
-            self.id as i32,
-            self.can_stand_in(),
-            self.can_stand_on(),
-            self.visibly_transparent(),
-            self.astar_weight as i32,
-            self.visible_range,
-        )
-    }
 }
 
 /// Packed fixture tile data. ~18 bytes vs 48 bytes tuple.
@@ -156,28 +133,14 @@ impl FixtureTileData {
     pub fn can_stand_on(&self) -> bool {
         self.flags & 0b010 != 0
     }
-
-    pub fn from_tuple(tuple: (i32, bool, bool, [u32; 8])) -> Self {
-        Self::new(tuple.0 as u8, tuple.1, tuple.2, tuple.3)
-    }
-
-    pub fn to_tuple(&self) -> (i32, bool, bool, [u32; 8]) {
-        (
-            self.id as i32,
-            self.can_stand_in(),
-            self.can_stand_on(),
-            self.visible_range,
-        )
-    }
 }
 
-/// 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)]
+/// Tile map using FxHashMap for fast lookups. No Arc wrapper - single-threaded access.
+#[derive(Resource, Default)]
 pub struct TileMap {
-    pub floor_tiles: Arc<AHashMap<IVec3, FloorTileData>>,
-    pub fixture_tiles: Arc<AHashMap<IVec3, FixtureTileData>>,
-    pub item_tiles: Arc<AHashMap<IVec3, Vec<u32>>>,
+    pub floor_tiles: FxHashMap<IVec3, FloorTileData>,
+    pub fixture_tiles: FxHashMap<IVec3, FixtureTileData>,
+    pub item_tiles: FxHashMap<IVec3, Vec<u32>>,
 }
 
 impl TileMap {
@@ -207,34 +170,26 @@ impl TileMap {
 
     #[inline]
     pub fn insert_floor(&mut self, pos: IVec3, tile: FloorTileData) {
-        Arc::make_mut(&mut self.floor_tiles).insert(pos, tile);
+        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);
+        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);
+        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)
+        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)
+        self.floor_tiles.get_mut(pos)
     }
 }