## Why
Paginated forks can retain a `history_base` pointer to another rollout. Deleting or compressing either side of that relationship can leave the fork without the rollout history it references.
## What changed
- Add a rollout reference index that scans active, archived, and compressed rollout metadata.
- Reject deletion when a rollout is still referenced by a fork outside the requested deletion set.
- Delete spawned thread subtrees as one store operation so internal references do not block deleting the complete subtree, while external references preflight the request before any files are removed.
- Skip referenced rollouts, fork-pointer rollouts, and rollouts with unreadable metadata during background compression.
## Testing
- Cover reference indexing across active, archived, compressed, duplicate, and self-referencing rollouts.
- Verify deletion rejects external references without partially deleting a subtree and permits batch deletion of internally referenced threads.
- Verify compression preserves fork pointer chains and continues past unreadable metadata.
GitOrigin-RevId: a8672727f8706e743d7fb3ca895a1d9ceac98a3c
