## Why
Completed voice captions could disappear before their queued history insertion ran. Interleaved user and assistant updates could also hide the other speaker's caption and restart its animation.
## What changed
- Preserve separate live caption cells for both speakers, rendering the user above the assistant without restarting settled text.
- Render completed captions while history insertion is pending, then move them into history in one app event. Keep them visible when agent streaming delays insertion.
- Invalidate the transcript overlay when an interleaved caption is cleared.
## Testing
Add regression tests for caption visibility through deferred history insertion, settled animations with either speaker arriving first, and empty caption completion during active and stopping sessions. Update replay coverage to exercise the app's history handoff.
GitOrigin-RevId: 31de6fc74b6dc7ff148c554e3176bf3ee92dced9
944d6fd1ba