fix: insert_floor syncs ChunkData, rabbit replaces dug tile with air

- TileMap::insert_floor now syncs ChunkData bits (set_floor_tile) so is_standable
  returns correct values for tiles inserted via insert_floor. Previously the
  HashMap was updated but ChunkData remained stale, causing is_standable to
  return false for valid tiles.
- rabbit_dig_system inserts air tile at the dug position after remove_floor.
  Without this, the HashMap entry is absent and ChunkData bits stay 0 (both
  false), making is_standable return false at that position. The entity then
  falls through multiple levels. Air's can_stand_in=true means the space is
  passable — the entity falls through it and lands on solid ground below.

src/entities/livestock/rabbit.rs

@@ -1,11 +1,11 @@
-use crate::config::GameConfig;
+use crate::config::{GameConfig, TileRegistry};
 use crate::constants::ITILE_SIZE;
 use crate::constants::TILE_SIZE;
 use crate::constants::*;
 use crate::entities::shared_components::Ambulatory;
 use crate::game::SpawnDelay;
 use crate::world::tiles::visibility::TileOcclusionEvent;
-use crate::world::tiles::{TileChangedEvent, TileMap};
+use crate::world::tiles::{FloorTileData, TileChangedEvent, TileMap};
 use crate::world::VisibleGameEntity;
 use bevy::prelude::*;
 use bevy_rand::prelude::*;
@@ -169,6 +169,24 @@ pub fn rabbit_dig_system(
         );
 
         let removed = tilemap.remove_floor(&below_pos);
+
+        // Replace the dug tile with air. The slot must become passable — if the entry
+        // is simply absent, ChunkData bits remain 0 (impassable), and is_standable
+        // returns false. Air has can_stand_in=true so entities can fall through it.
+        if removed.is_some() {
+            let air = TileRegistry::global().floor("air");
+            tilemap.insert_floor(
+                below_pos,
+                FloorTileData::new(
+                    air.id,
+                    air.can_stand_in,
+                    air.can_stand_on,
+                    air.transparent,
+                    air.astar_weight,
+                    [0; 8],
+                ),
+            );
+        }
         println!(
             "[DIG] remove_floor({:?}) => {}",
             below_pos,