Extract player/mod.rs, playback_engine.rs, and streaming_source.rs to
qbz-player crate. Also adds settings.rs and visualizer module to
qbz-audio (Tauri-specific code removed, core functionality retained).
Changes:
- Add settings.rs to qbz-audio with AudioSettings, AudioSettingsStore
- Add visualizer/ to qbz-audio with RingBuffer, TappedSource, VisualizerTap
- Add player module files to qbz-player
- Update all imports from crate::audio to qbz_audio
- Add serde_json dependency to qbz-audio
Full workspace compiles with cargo check.