- Add abogen/domain/audio_buffer.py with core audio operations:
- create_silence(): create silence audio buffer
- mix_audio(): mix source into target buffer with auto-resize
- normalize_audio(): normalize to prevent clipping
- ensure_buffer_size(): extend buffer to minimum size
- concatenate_audio(): join multiple audio buffers
- audio_duration(): calculate duration from samples
- samples_for_duration(): calculate samples from duration
- SAMPLE_RATE constant (24000)
- Update abogen/pyqt/conversion.py:
- Import and use create_silence for chapter silence
- Use mix_audio for subtitle file mixing
- Use normalize_audio for clipping prevention
- Use create_silence for padding in subtitle processing
- Update abogen/webui/conversion_runner.py:
- Import and use create_silence in append_silence
- Replace np.zeros with domain function
- Add tests/test_audio_buffer.py with comprehensive unit tests
