Make audio callback allocation-free

[balloob-travel]

Summary

• copy queued PCM directly into PortAudio's output buffer instead of building temporary callback buffers
• reuse scratch storage for silence and duplicated frames while preserving sync-correction behavior
• add pytest coverage for the callback fast path and drop-correction path, and include pytest in test extras

Testing

• uv run pre-commit run --files pyproject.toml sendspin/audio.py tests/test_audio.py
• python -m pytest tests/test_audio.py