fix: keep a tile's monster flag while another monster or pet stands on it

- Add MapManager.RemoveMonsterFlag, which clears TileFlag.Monster only when no other live creature or pet on the map still stands on the tile

- Route every monster and pet flag removal (moves, jumps, Dash push, despawns, reloads, /remove) through it

- Pet catch-up teleport now clears its old tile flag and sets the new one

- Add regression tests for a shared tile and the pet catch-up