- 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
popertots
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