feat: stage 2 collision, TileChangedEvent, path invalidation, rabbit digging

- Stage 2 collision: convoy skip when entities move same direction (dot>0.7),
  E/S yield rule for crossings, W/N right-of-way, head-on unchanged
- TileChangedEvent (Message) + PathfindingDirtyChunks (Resource) in new tile_changed module
- collect_pathfinding_dirty_chunks / invalidate_paths_on_tile_change /
  clear_pathfinding_dirty_chunks in PathfindingPlugin FixedUpdate chain
- TileMap::remove_floor clears HashMap + ChunkData bitsets + tile_ids
- RabbitDigTimer component: rabbits dig floor below every 5s, fires TileChangedEvent
  and TileOcclusionEvent for path invalidation + rendering

src/world/tiles/tile_changed.rs

@@ -0,0 +1,17 @@
+//! Tile change events and pathfinding dirty chunk tracking.
+//!
+//! This module provides types for tracking tile modifications and
+//! invalidating pathfinding data when terrain changes.
+
+use bevy::prelude::*;
+use bevy_platform::collections::HashSet;
+
+#[derive(Message, Clone, Copy)]
+pub struct TileChangedEvent {
+    pub pos: IVec3,
+}
+
+#[derive(Resource, Default)]
+pub struct PathfindingDirtyChunks {
+    pub chunks: HashSet<IVec2>,
+}