## Why
Archiving threads reads metadata from every active rollout, including unrelated compressed files.
## What changed
Add `RolloutReferenceIndex::scan_unarchived_threads` and use it when archiving. Filter canonical filenames by the requested thread IDs before reading contents, while retaining metadata-based ownership checks.
## Testing
Add tests covering reverted rollouts, requested descendants, compressed rollouts, exclusion of archived and unrelated files, and candidate ownership checks.
GitOrigin-RevId: a64f7fa25e5493ed20312f1d8216422752272bae