fix: rabbit dig occlusion refresh, bounds guard panic, world floor clamp

- Fire TileOcclusionEvent for 27-tile neighbourhood (3x3x3) so tiles above
  the dug tile recalculate visibility and stop rendering black
- Clamp gravity in movement to world minimum z — rabbits digging the
  floor below them now stop at world floor instead of falling past it
- Assert bounds in remove_floor/remove_fixture before pos_to_index to
  panic with z/coords info instead of silently overflowing
- Hoist ITILE_SIZE, CHUNK_SIZE, Z_BELOW, Z_ABOVE to module-level
  imports in tilemap.rs

src/entities/shared_systems/pathfinding.rs

@@ -609,7 +609,12 @@ pub fn movement(
         .for_each(|(mut ambulatory, mut transform)| {
             let current_pos = transform.translation;
             if !is_standable_tile(&tilemap, current_pos.as_ivec3()) {
-                transform.translation.z -= TILE_SIZE;
+                // Don't fall below world minimum z — rabbits digging can leave no floor
+                // to stand on, but they should stop at the world floor rather than
+                // falling into oblivion and panicking pos_to_index overflow.
+                if transform.translation.z > -(crate::world::chunks::Z_BELOW as f32) * TILE_SIZE {
+                    transform.translation.z -= TILE_SIZE;
+                }
                 ambulatory.current_path = None;
                 ambulatory.target = None;
                 return;