## Why
Building a resume-picker preview for a legacy session should not require loading its full thread history.
## What changed
- Read up to six preview lines by scanning at most 1 MiB from the end of a local legacy rollout.
- Fall back to the existing history hydration path when the local file is unavailable, the bounded scan is incomplete, or the tail contains a rollback.
- Keep the initial `thread/read` request turn-free while loading the preview.
## Testing
Added coverage for compaction, rollbacks, oversized records, exhausted scan budgets, and app-server request parameters.
GitOrigin-RevId: a8950f03d8b56323f268fd3355c6a38a16cb9833
## What changed
- Scan legacy and paginated transcript items newest-first and stop after collecting the six newest nonblank preview lines.
- Keep paging through bounded paginated history when newer items do not produce visible preview text, while preserving chronological display order and assistant markdown rewrites.
- Move transcript preview loading into a focused module and add coverage for reverse scanning, legacy/paginated parity, and paging past invisible items.
GitOrigin-RevId: 576afe37d034b32db363691bd4f401815fffaea7
Charlie Marsh