## Why
`thread/fork` rejected threads using paginated history, so they could not use the same fork workflow as legacy threads.
## What changed
- Create paginated forks by referencing a frozen source-history prefix while persisting only child-owned records in the new rollout.
- Support latest, `lastTurnId`, and `beforeTurnId` boundaries, including inherited lineage, model-context reconstruction, interruption markers, and approval-reviewer inheritance.
- Make paginated reads and occurrence search traverse referenced history while excluding source records added after the fork.
- Coordinate fork preparation with archive and delete operations, and materialize compressed rollouts before they become fork references.
## Testing
Added app-server and thread-store coverage for boundary selection, active turns, inherited reads and search, compressed lineage, and concurrent source lifecycle operations.
GitOrigin-RevId: 66052d69517d7fb97a3b7817ab77a39c5ad995fd
05f000263b