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/livestock/rabbit.rs

@@ -128,9 +128,19 @@ pub fn rabbit_dig_system(
 
         if tilemap.remove_floor(&below_pos).is_some() {
             tile_changed.write(TileChangedEvent { pos: below_pos });
-            occlusion.write(TileOcclusionEvent {
-                tile_position: below_pos,
-            });
+
+            // Refresh visibility for the dug tile and all 26 neighbours.
+            // Removing a tile changes what is visible from every adjacent position.
+            for dz in -1..=1i32 {
+                for dy in -1..=1i32 {
+                    for dx in -1..=1i32 {
+                        occlusion.write(TileOcclusionEvent {
+                            tile_position: below_pos
+                                + IVec3::new(dx * ITILE_SIZE, dy * ITILE_SIZE, dz * ITILE_SIZE),
+                        });
+                    }
+                }
+            }
         }
     }
 }