## Why
Analytics need to associate subagent activity with the top-level turn that
caused it, while avoiding a stale association when steering makes that origin
ambiguous.
## What changed
- Add `root_turn_id` to turn and tool-item analytics events.
- Read the root from live turn metadata when each event is built, reporting
`null` when the root is unknown or ambiguous.
- Propagate the metadata through control-tool and code-mode completion facts so
correlated and background tool events retain the correct root.
## Testing
- Cover root IDs for top-level turns, subagent turns, correlated code-mode
calls, and tool events emitted after turn completion.
- Verify that queued and subsequent events report `null` after the live root is
invalidated.
GitOrigin-RevId: 42fd76541c3c716cec74787d47b50adb2d22e6d2