Handle missing TRES table

[NessieCanCode]

Summary

• attempt to read TRES mapping from tres and fallback to tres_table for older schemas
• add regression tests for fallback and missing table

Testing

• for f in test/unit/*.test.py; do PYTHONPATH=src python "$f" >/tmp/test.log 2>&1; cat /tmp/test.log; done
• make check >/tmp/check.log 2>&1 && tail -n 20 /tmp/check.log

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https://chatgpt.com/codex/tasks/task_e_6894cfc0cf1c8324b022fc48ef869046