fix(lancedb): treat a lance spill failure as retryable

[gloryfromca]

Summary

The cascade worker retries ExternalServiceError with backoff and files any other exception as unrecoverable (mark_failed(retryable=False), surfaces in cascade fix). lancedb raises LanceError(IO): Execution error: Spill has sent an error — DataFusion's sort/merge spill to the OS temp dir failing mid-query — as a bare RuntimeError, so every occurrence marked an md file permanently failed.

On the Windows soak box (ThinkPad, nine concurrent clients) it fired 187 / 180 / 34 times across the three soak runs, never on an idle box, and the same rows project fine on the next attempt; roughly 200 files per run were left needing a manual cascade fix. The run-1 report attributed the unrecoverable count to fuzzed malformed markdown only; this class was hiding under it.

Fix: translate that one phrase into VectorStoreBusyError inside the read deadline wrapper (LanceRepoBase._deadline) — the observed path is worker → find_by_md_path / delete_by_md_path → repository → lancedb query. Other IO errors (No space left, missing file) propagate unchanged and stay permanent.

Area

• architecture (storage)

Verification

• tests/unit/test_core/test_persistence/test_lancedb/test_transient_execution_errors.py: the phrase becomes VectorStoreBusyError; an unrelated IO error does not; the marker predicate. Emptying the marker tuple fails the translation test.
• test_core/test_persistence/test_lancedb + test_cascade: green. ruff green.
• Not reproduced locally: the spill failure only appears on the loaded Windows box; the soak on the integration branch with this change is the live check (its unrecoverable_total should drop to the fuzz-only baseline).

Checklist

• tests added, mutation checked

Notes for Reviewers

Why not a broader LanceError(IO) match: disk-full and vanished-file errors carry the same prefix and must stay permanent. Why not in _locked too: the write path was not where the error was observed; widening is a one-line follow-up if a soak shows it there.

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