## Why
Legacy rollouts can contain historical rollbacks, compaction checkpoints, and
subagent copies of parent history. Migrating those records verbatim can change
the visible conversation or the model context used when a thread resumes, and
can retain large amounts of duplicated subagent history.
## What changed
- Plan and replay historical rollbacks by logical turn, preserving surviving
lifecycle events and the correct compaction anchor for future resumes.
- Migrate subagent rollouts from a safe bounded replay when possible and record
the history boundary so copied parent context is not projected as child turns.
- Include archived and compressed rollouts while keeping their storage form and
location intact.
- Tolerate malformed, partial, oversized, and empty JSONL records during the
migration scan.
## Testing
Add coverage for rollback and compaction combinations, subagent replay bounds,
archived and compressed rollouts, recovery, concurrent maintenance, and damaged
JSONL input.
GitOrigin-RevId: 3948e16d43f75df8571b0a92af12a181d1d08fdf
## 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- Cover frozen-prefix reverse scans and nested lineages whose root rollout has been archived.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
## What changed
- Add a generic `ReverseJsonlScanner` that reads seekable JSONL data from the
end in bounded chunks, skips blank records, and reports malformed records
without stopping the scan.
- Use the scanner for reverse session-index lookups while preserving the
existing behavior of ignoring invalid entries.
## Testing
- Add coverage for unterminated final records, malformed JSON, blank lines,
chunk boundaries, and records spanning multiple chunks.
- Verify session-index lookup can skip an invalid record and still find valid
entries on either side of it.
GitOrigin-RevId: 66255f48492f74726d4ac4831cdc82f748de17d6
Owen Lin