## Description
This PR moves review-mode markers onto canonical `TurnItem` lifecycle:
- `TurnItem::EnteredReviewMode`
- `TurnItem::ExitedReviewMode`
Core now emits `ItemStarted` / `ItemCompleted` for both. The completed
items map back into the existing `EnteredReviewMode` /
`ExitedReviewMode` events, so raw core event consumers and legacy
rollout persistence keep seeing the old events.
This is the compatibility layer needed before paginated rollouts persist
review markers as `ItemCompleted(TurnItem)`.
## Why
Review markers were one of the remaining app-server thread items created
directly from legacy events. Giving them canonical items lets paginated
history persist stable turn/item IDs without changing legacy rollouts.
## What changed
- Added canonical review-mode `TurnItem`s and switched review flow to
emit their lifecycle.
- Added completed-item → legacy review event mappings with stable
turn/item IDs.
- Switched app-server live notifications to the generic canonical item
path and kept legacy replay compatible with old payloads.
- Updated `ThreadHistoryBuilder` to replay canonical review items even
though review turns still do not emit `TurnStarted`.
Owen Lin
23aac925e7