* fix: persist run duration in checkpoints for history reads
* fix(checkpoint): harden run duration persistence
* fix(checkpoint): persist run durations in metadata
* fix(checkpoint): address review findings for run duration persistence
- Add valid_duration_entry() shared validation helper (worker.py)
- Rename _persist_run_durations -> persist_run_durations as public API
- Import public persist_run_durations and valid_duration_entry in threads.py
- Use BackgroundTasks for lazy backfill write to avoid blocking history reads
- Add TODO about O(runs) growth of run_durations in checkpoint metadata
- Document REGENERATE_HISTORY_RAW_SCAN_LIMIT doubling assumption
* fix(checkpoint): replace pruning TODO with justification
Accumulated run_durations overhead (~50 bytes/run_id) is negligible compared
to messages channel blobs; no pruning strategy is needed.
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
