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/config.rs

@@ -1,6 +1,8 @@
 use bevy::prelude::Resource;
 use serde::Deserialize;
+use std::collections::HashMap;
 use std::fs;
+use std::sync::OnceLock;
 
 #[derive(Debug, Deserialize, Clone, Resource)]
 pub struct GameConfig {
@@ -21,3 +23,51 @@ impl GameConfig {
         toml::from_str(&config_str).expect("Failed to parse config.toml")
     }
 }
+
+static TILE_REGISTRY: OnceLock<TileRegistry> = OnceLock::new();
+
+#[derive(Debug, Deserialize, Clone)]
+pub struct TileRegistry {
+    pub floor_tiles: HashMap<String, FloorTileDef>,
+    pub fixture_tiles: HashMap<String, FixtureTileDef>,
+}
+
+#[derive(Debug, Deserialize, Clone, Copy)]
+pub struct FloorTileDef {
+    pub id: u8,
+    pub can_stand_in: bool,
+    pub can_stand_on: bool,
+    pub transparent: bool,
+    pub astar_weight: u8,
+}
+
+#[derive(Debug, Deserialize, Clone, Copy)]
+pub struct FixtureTileDef {
+    pub id: u32,
+    pub solid: bool,
+}
+
+impl TileRegistry {
+    pub fn load() -> Self {
+        let config_str = fs::read_to_string("tiles.toml").expect("Failed to find tiles.toml");
+        toml::from_str(&config_str).expect("Failed to parse tiles.toml")
+    }
+
+    pub fn global() -> &'static Self {
+        TILE_REGISTRY.get_or_init(|| Self::load())
+    }
+
+    pub fn floor(&self, name: &str) -> FloorTileDef {
+        *self
+            .floor_tiles
+            .get(name)
+            .unwrap_or_else(|| panic!("Unknown floor tile: {}", name))
+    }
+
+    pub fn fixture(&self, name: &str) -> FixtureTileDef {
+        *self
+            .fixture_tiles
+            .get(name)
+            .unwrap_or_else(|| panic!("Unknown fixture tile: {}", name))
+    }
+}