## What changed
- Add fractional Unix creation times to locally authored user, developer, agent, and tool-output items when they enter durable conversation history.
- Preserve any creation time already supplied with an item across subsequent requests, remote compaction, and resumed history.
## Testing
- Cover timestamp assignment and preservation in session, client, tool, agent-spawn, and remote-compaction tests.
GitOrigin-RevId: 431d1770b9746005c808bc0a43953845166d68ad