## Why
Archiving a thread only needs to locate rollout files that have not already
been moved, so reading the entire rollout archive on every request is
unnecessary.
## What changed
- Add `RolloutReferenceIndex::scan_unarchived` to scan only the active
`sessions` directory.
- Use the active-only index when archiving threads while retaining full scans
for operations that require archived-history reference counts.
- Cover discovery of compressed and uncompressed active rollouts and exclusion
of archived and unrelated rollouts.
GitOrigin-RevId: 46367127578f20bfa46a8db5c6f2b7f59d29f7a9