perf(transaction): cache serialized records to avoid double serialization

[tbhb]

Summary

• Cache serialized records in Transaction._buffer_serialized during put() and reuse at commit() time, eliminating redundant serialization
• Remove unused start_mtime parameter from _commit_transaction_buffer
• Add key length validation to Transaction.delete() and Table.delete() for consistency with put() methods
• Extract validate_key_length() helper to _keys.py to reduce code duplication

Test plan

• All existing tests pass (632 tests)
• Added tests for key length validation in delete operations
• Type checking passes (basedpyright)
• Linting passes (ruff)
• Pre-commit hooks pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[codspeed-hq]

CodSpeed Performance Report

Merging #10 will not alter performance

_{Comparing cache-serialized-records-in-tx (ca60cad) with main (75ed41c)¹}

Summary

✅ 173 untouched
⏩ 1 skipped²

Footnotes

1. No successful run was found on main (f9bbece) during the generation of this report, so 75ed41c was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

2. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩