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/entities/livestock/rabbit.rs

@@ -134,14 +134,15 @@ pub fn rabbit_dig_system(
         if tilemap.remove_floor(&below_pos).is_some() {
             tile_changed.write(TileChangedEvent { pos: 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 {
+            // Refresh the full column below the dig and its XY neighbours.
+            // calculate_visibility traces up to Z_TOTAL+Z_BELOW+1 tiles above each tile,
+            // so removing one tile can affect visibility arbitrarily deep below.
+            for dz in 0..=crate::world::chunks::Z_BELOW as i32 {
                 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),
+                                - IVec3::new(dx * ITILE_SIZE, dy * ITILE_SIZE, dz * ITILE_SIZE),
                         });
                     }
                 }