## What changed
- Combine file-backed rollouts and buffered rollout prefixes into `rollouts.tar.gz`, keeping diagnostics as separate attachments.
- Bound the archive and its upload envelope by the existing size limit, skip invalid or duplicate archive filenames, and fall back to individual rollout attachments if archiving fails.
- Add `rollout_archive_filename` to the report thread index so rollout filenames identify archive entries or fallback attachments.
## Testing
Add an upload test that extracts the archive and verifies the filenames and contents of both a compressed file-backed rollout and a buffered rollout. Update the thread index test to check the archive filename.
GitOrigin-RevId: 973da6eec2249b04ef7992b5e1e4fd27f044b80f
