Close the tile stream opened during existence checks and validate parent-map fallback directly. Track each tile's successful model acquisitions so failed or partial loads cannot release references owned by dynamic objects or other tiles.
Add production-method regression tests covering file descriptor stability, transient acquisition failures, shared references, duplicate loads, partial files and parent-map overlaps. Reproduced both original defects on 6f8f59ec6d; build and test-server verification passed.
