## Why
Compaction replaces the model history window, but host-verified user answers may
still be needed for the rest of the owning thread.
## What changed
- Add bounded, model-invisible retained context for verified
`request_user_input` answers.
- Persist retained context in rollout events and compaction checkpoints, and
restore it when a thread resumes.
- Remove answers when their source turns are rolled back, and prevent
parent-local answers from being inherited by child forks.
## Testing
Add lifecycle coverage for deduplication and size limits, compaction and resume,
rollback, rollout migration, and child forks.
GitOrigin-RevId: c32900d3e8120a9899b71f0576683acd67a4b898