## What changed
- Resolve the full rollout lineage when loading model context for paginated threads.
- Reverse-scan each lineage segment up to its recorded byte boundary, stopping at segment metadata and preserving the requested thread's canonical session metadata.
- Add a bounded reverse-scanner constructor for scanning a frozen JSONL prefix.
## Testing
- Cover frozen-prefix reverse scans and nested lineages whose root rollout has been archived.
GitOrigin-RevId: 8b97da43a263b7bd09cb7883450bb19eba5f195f