adk-2.0 producer: capture or explicitly defer actions.route / render_ui_widgets / rewind_before_invocation_id

Parent tracker: #190 (v15 contract).
Wave: 2 — Producer event rows.

## Contract

`EventActions` carries three more ADK 2.0 surfaces that the umbrella has not yet committed on:

- `actions.route` (`event_actions.py:114`) — workflow graph edge routing value.
- `actions.render_ui_widgets` (`event_actions.py:117`) — UI widget side effects.
- `actions.rewind_before_invocation_id` (`event_actions.py:111`) — invocation rewind requests.

Decide per-field: capture under `attributes.adk.route` / `attributes.adk.render_ui_widgets` / `attributes.adk.rewind_before_invocation_id`, or explicitly defer with a note in this issue + #190 explaining why.

## Acceptance

- [ ] Each of the three fields has a written decision: capture or explicit defer.
- [ ] For captured fields: serialization shape documented + at least one fixture row.
- [ ] For deferred fields: rationale documented and re-evaluation trigger noted (e.g. "revisit when UI widget analytics requested").

## References

- #190 (v15 producer §10).
- `src/google/adk/events/event_actions.py:111-118`.