Replace rodio with cpal for AudioSink

- Replace rodio dependency with cpal in pmoaudio/Cargo.toml
- Add AudioSink node using cpal for direct hardware access
- Add SharedBuffer for async/callback communication
- Convert all audio formats to F32 for cpal
- Improve latency and control over audio stream
- Add WHY_CPAL.md explaining the technical choice
- Update INSTALL_NOTES.md with ALSA requirements
- Export AudioSink in lib.rs and mod.rs

Benefits:
- Minimal latency (no extra layers)
- Direct hardware control
- Lighter binary (~3.8 MB less)
- Same ALSA dependency as rodio on Linux
- Cross-platform (ALSA/JACK on Linux, CoreAudio on macOS, WASAPI on Windows)

pmoaudio/Cargo.toml

@@ -19,6 +19,7 @@ soxr = "0.6.0"
 bytemuck = "1.24.0"
 reqwest = { version = "0.12", features = ["stream"] }
 tracing = "0.1"
+cpal = "0.15"
 
 [dev-dependencies]
 tokio-test = "0.4"