[Feature]: Add exporter model support for extension audio events

[crbender]

• I have searched the existing issues

Is your feature request related to a problem?

Yes. Exporter audio model currently does not have a first-class input for extension-triggered audio events.

Describe the solution you'd like

Add exporter input model support for extension audio events so captured events can be mixed in both classic and modern exporter paths.

Describe alternatives you've considered

Mapping all extension sounds into generic user audioRegions. Possible as a fallback, but less explicit and harder to evolve for extension-specific controls.

Additional context

Need data model that carries event timing and gain cleanly through export pipeline.

Acceptance criteria

1. Export config accepts extension audio event input.
2. Extension event input supports timestamp and gain.
3. Both exporter paths can consume the same normalized event model.
4. Unit tests cover model normalization and serialization boundaries.

[crbender]

Tracking under parent issue #572 for sequencing and rollout context.