[Feature]: Add regression and integration coverage for extension audio export parity

[crbender]

• I have searched the existing issues

Is your feature request related to a problem?

Yes. This work touches high-risk export/audio flows and needs strong regression coverage.

Describe the solution you'd like

Add targeted test coverage for extension audio export parity:

1. Unit tests for extension host capture mode
2. Integration tests for preview/export event parity
3. Export audio assertions around expected click timestamps
4. Regression tests for no-extension and missing-asset cases

Describe alternatives you've considered

Relying only on manual checks. Rejected, too risky for future export refactors.

Additional context

Coverage should focus on behavior confidence, not just line coverage.

Acceptance criteria

1. New tests are deterministic and stable in CI.
2. Tests prove click sounds are included in exports when triggered.
3. Tests prove no behavior change when extension audio is absent.
4. Test docs include reproducible local run commands.

[crbender]

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