fix: treat absent tiles as air in visibility, full-column occlusion refresh

- calculate_visibility: None from floor_tiles.get() now breaks touches_air=true.
  Absent tile = open space = air for visibility purposes. Previously, removed
  tiles left their below-neighbours with all-zero visible_range (black).
- rabbit_dig_system: fire TileOcclusionEvent for full Z_BELOW+1 level column
  below the dug tile + 8 XY neighbours each, not just 3x3x3. calculate_visibility
  traces up to Z_TOTAL+Z_BELOW+1 tiles above each tile, so removing one tile
  can expose visibility arbitrarily deep below.

src/world/tiles/visibility.rs

@@ -144,7 +144,8 @@ pub fn calculate_visibility(pos: IVec3, tilemap: &TileMap) -> [u32; 8] {
                     );
                     match tilemap.floor_tiles.get(&neighbor_pos) {
                         Some(tile) if tile.id == 0 => break 'neighbor_check true,
-                        None => {}
+                        // Absent tile means open space — treat as air for visibility.
+                        None => break 'neighbor_check true,
                         _ => {}
                     }
                 }