fix: retry final log flush

[vycdev2]

Summary

• retry queued log persistence up to three times when the background writer performs its final shutdown flush
• retain the batch between attempts so a one-off database failure does not silently discard shutdown logs
• add a regression test covering a transient persistence failure during final flush

Verification

• dotnet build — passed (with the existing NU1903 SQLitePCLRaw advisory warning)
• dotnet test --no-restore --verbosity minimal — passed: 429 passed, 1 skipped
• python3 -m unittest tools.test_generate_commands_md — passed: 3 tests
• targeted shutdown regression repeated 10 times — passed each run
• git diff --check — passed

Risk

• Low: the change is limited to shutdown flushing; persistent failures remain bounded to three attempts and can add at most two 250 ms retry delays.

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.