## Why
Buffered thread snapshots can contain both streaming deltas and the completed
assistant item. Replaying both reconstructs an already-finished stream instead
of restoring its finalized transcript entry.
## What changed
- Omit superseded assistant deltas from replay snapshots when a matching
completed item follows them.
- Preserve unfinished or unrelated deltas and treat other events as replay
barriers so text needed before tools or prompts is not dropped.
- Render completed replay items directly as formatted history cells while
preserving message order, separators, and composer drafts.
## Testing
Added replay tests covering completed, unfinished, unrelated, and
barrier-separated deltas, plus deterministic Markdown rendering and draft
preservation.
GitOrigin-RevId: 2870b09a04b32c0a4c0e9ab76970a8ebc3f5a7d4
Benjamin Carlsson
e6ff749506