fix: remove_floor clears only stand_on_floor, not stand_in_floor

Clearing stand_in_floor when removing a floor tile made the dug position
impassable. An entity falling into the dug slot found (false || false) && true
= false and kept falling. Now only stand_on_floor is cleared — the space
reverts to air (passable), and only the tile above loses its platform.

src/world/tiles/tilemap.rs

@@ -316,7 +316,11 @@ impl TileMap {
             let idx = ChunkData::pos_to_index(lx, ly, z);
             let word = idx / 32;
             let clear_mask = !(1u32 << (idx % 32));
-            chunk.stand_in_floor[word] &= clear_mask;
+            // Only clear stand_on_floor. Removing a floor tile means the space becomes
+            // air — stand_in_floor should stay true so entities can pass through.
+            // Only stand_on_floor needs clearing: the tile above can no longer stand on
+            // a tile that doesn't exist. Clearing stand_in_floor made dug positions
+            // impassable, causing entities to fall through multiple levels.
             chunk.stand_on_floor[word] &= clear_mask;
             chunk.tile_ids[idx] = 0;
         }