## Why
Replaying delivered assistant messages allocates new retained-source revisions instead of preserving those captured in live history.
## What changed
- Capture retained sources when recording assistant deliveries, persist them in rollout metadata, and reuse their revisions during replay.
- Add optional `next_item_ordinal` compaction metadata and a `next_model_id_ordinals` helper to recover item and window allocation positions from checkpoints and subsequent records. Ignore allocation metadata that refers to other threads.
## Testing
Extend tests to compare live and replayed delivery sources, preserve revisions through an older reader's rewrite, and verify ordinal recovery from checkpoints and subsequent local items.
GitOrigin-RevId: cfdeebe8e13b1700b34b9c98696880a90a99ace5