## What changed
- Add optional start and completion timestamps to `ThreadHistoryItemChange`.
- Populate them from canonical `ItemCompleted` records during stateless thread history projection.
- Treat a zero completion timestamp as absent, and leave timestamps unset for legacy builder events that do not provide them.
## Testing
- Cover projected item timestamps, including missing start times and zero completion times.
GitOrigin-RevId: 2187530f489ad3d86b6ab65faaaab1eab5357f1b
## What changed
- Add an optional `started_at_ms` field to `ItemCompletedEvent`, retaining
compatibility with older persisted events that lack it.
- Track the first start timestamp for each in-flight item and attach it when
emitting and persisting the completion event. If no start was recorded, use
the completion timestamp as a fallback.
- Emit a complete start/completion lifecycle for subagent activity items.
## Testing
- Cover concurrent item timing, repeated starts, turn-boundary cleanup, the
missing-start fallback, subagent activity, and persisted web-search events.
GitOrigin-RevId: e7cec9c4f1ef6ba67f287e81fb4d7d856fcf87a7
## Why
Paginated thread history needs durable ordering so consumers can process a rollout suffix without rebuilding all earlier history.
## What changed
- Add optional, zero-based ordinals to `RolloutLine` records in paginated rollouts while leaving legacy rollout serialization unchanged.
- Continue ordinals from the last valid record when appending or resuming, including after gaps or an incomplete tail, and reject overflow without appending.
- Add a stateless `project_rollout_line` helper that maps canonical turn lifecycle and completed-item records into thread-history change sets.
## Testing
- Cover ordinal assignment, legacy compatibility, resume and tail recovery, overflow handling, and thread-history projection for completed, failed, and interrupted turns.
GitOrigin-RevId: 3a9bb6cd2a1f674a9f154342e96e9aa3d8330781
Owen Lin