Flatten attachment paths with message-prefixed filenames

- Write projected attachments under the thread directory using `messageSegment-fileName`
- Avoid nested per-message folders while preserving unique, deterministic file paths
- Update projection pipeline tests to match new attachment URLs and on-disk paths
b52518f85fJulius Marminge committed on 2/28/2026, 5:30:46 AM· committed by Julius Marmingeparent3e99019
2 files changedLine totals unavailable