[Feature]: Add extension host export audio capture lifecycle

[crbender]

• I have searched the existing issues

Is your feature request related to a problem?

Yes. Extension playSound() currently uses runtime playback only, which is not consumable by the export mixer.

Describe the solution you'd like

Add export capture lifecycle APIs to extension host and capture extension audio trigger events for export.

Proposed host lifecycle:

1. beginExportAudioCapture()
2. endExportAudioCapture()
3. getCapturedAudioEvents()

Captured event shape should include at minimum:

• extensionId
• resolvedAssetPath
• timeMs
• volume

Preview playback behavior should remain unchanged.

Describe alternatives you've considered

Using ad-hoc global state in exporter only. Rejected, too brittle and hard to test.

Additional context

This is a dependency for export parity work.

Acceptance criteria

1. Preview mode keeps current audible behavior.
2. Export capture mode records deterministic event stream.
3. Event stream order is stable and timestamp-sorted.
4. Unit tests cover mode switching and event capture correctness.

[crbender]

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