fix: snap-to-top for above-world spawn, guard rabbit dig at world floor

- Entities spawn at z=35*TILE=560 but Z_ABOVE max is 15*TILE=240. Snap to
  Z_ABOVE*TILE instead of falling one tile per tick through unloaded space.
- Skip rabbit dig when below_pos would be outside ChunkData z-bounds,
  preventing remove_floor assert panic.

src/entities/livestock/rabbit.rs

@@ -126,6 +126,11 @@ pub fn rabbit_dig_system(
         let entity_pos = transform.translation.as_ivec3();
         let below_pos = IVec3::new(entity_pos.x, entity_pos.y, entity_pos.z - ITILE_SIZE);
 
+        // Don't dig below world floor — would panic in remove_floor bounds check.
+        if below_pos.z < -(crate::world::chunks::Z_BELOW as i32 - 1) * ITILE_SIZE {
+            continue;
+        }
+
         if tilemap.remove_floor(&below_pos).is_some() {
             tile_changed.write(TileChangedEvent { pos: below_pos });