## Why
Realtime history should be recorded consistently for every Core host, including
when no app-server event listener is attached.
## What changed
- Move transcript segmentation, session boundaries, and backing-agent artifact
promotion into Core for paginated threads.
- Persist canonical realtime items through the thread store in event order and
emit dedicated history lifecycle events for hosts to present.
- Translate those Core events into the existing app-server realtime item
notifications without app-server persisting the items a second time.
## Testing
- Cover Core-only persistence across repeated sessions, ephemeral sessions,
accepted and rejected steering, typed input ordering, and artifact promotion.
- Verify app-server notifications correspond to the persisted timeline.
GitOrigin-RevId: 7cbef14129d77f6d6d7099b733be91f5279c55f5