test: Add hardware and macOS audio integration coverage

## What
Add a hardware/macOS integration test plan for microphone capture, BlackHole detection, system-audio fallback, and sample-format variants.

## Why
- `.planning/codebase/CONCERNS.md` marks hardware and OS integration coverage as a high-priority gap.
- Audio behavior depends on real CoreAudio devices, BlackHole installation, and `cpal` sample formats.
- Unit tests cannot catch failures that only occur on specific macOS versions or hardware.

## How
- Define a manual or opt-in automated audio test matrix for supported macOS versions.
- Include default mic, non-`f32` sample format devices where available, BlackHole installed/missing, and system-audio failure cases.
- Record expected UI warnings and logs for each scenario.
- Add lightweight helper commands or diagnostics to make test runs reproducible.