Browser test for microphone-triggered barge-in

## Summary

Pipeline interruption during TTS is unit-tested. Known limitation: microphone-triggered barge-in is not in Playwright.

## Context

- `docs/release/known-limitations.md`
- `src/voxforge/modules/livekit_gateway/application/session_runner.py`
- `tests/` unit coverage for TTS interruption / barge-in metrics
- Public demo Start talking path

## Scope

- [ ] Playwright (or feature test with fake media) that starts TTS, then injects mic/audio interrupt
- [ ] Assert interrupt is recorded (UI indicator or API/session flag)
- [ ] Must run in CI without a physical mic (use fake devices / Playwright permissions)

## Acceptance criteria

Document how to grant fake-audio in CI; skip cleanly if unsupported rather than flake.

Start with src/voxforge/modules/livekit_gateway/application/session_runner.py and the existing tests/ coverage for TTS interruption and barge-in metrics. Review the public demo Start talking path and the Playwright setup, then run the relevant tests before adding fake microphone input. Done means CI reliably records the interrupt, documents fake-audio permissions, and skips cleanly when unsupported.

Written by the indexing model from the issue text.