feat(kon): add audio crate — cpal capture, VAD, rubato resample, symphonia decode

- File decode via symphonia (mp3, aac, flac, wav, ogg) with mono mixdown
- Sinc interpolation resampling via rubato 0.15 (48kHz/44.1kHz → 16kHz)
- WAV I/O via hound (read/write with f32→i16 conversion)
- Microphone capture via cpal 0.17 (WASAPI on Windows) with mpsc channel output
- Voice activity detection via Silero VAD V5 (voice_activity_detector 0.2)
- Async decode_and_resample() for file transcription pipeline
- 3 tests passing (resample passthrough, duration preservation, WAV roundtrip)
- clippy clean, no ort version conflicts

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/audio/src/lib.rs

@@ -1,2 +1,13 @@
-// kon-audio: Microphone capture, voice activity detection, resampling,
-// file decoding, and WAV I/O.
+pub mod capture;
+pub mod concurrency;
+pub mod decode;
+pub mod resample;
+pub mod vad;
+pub mod wav;
+
+pub use capture::{AudioChunk, MicrophoneCapture};
+pub use concurrency::decode_and_resample;
+pub use decode::decode_audio_file;
+pub use resample::resample_to_16khz;
+pub use vad::SpeechDetector;
+pub use wav::{read_wav, write_wav};