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/concurrency.rs

@@ -0,0 +1,19 @@
+use std::path::Path;
+
+use kon_core::error::Result;
+use kon_core::types::AudioSamples;
+
+use crate::decode::decode_audio_file;
+use crate::resample::resample_to_16khz;
+
+/// Decode and resample an audio file on a blocking thread.
+/// Returns 16kHz mono AudioSamples ready for transcription.
+pub async fn decode_and_resample(path: &Path) -> Result<AudioSamples> {
+    let path = path.to_path_buf();
+    tokio::task::spawn_blocking(move || {
+        let audio = decode_audio_file(&path)?;
+        resample_to_16khz(&audio)
+    })
+    .await
+    .map_err(|e| kon_core::error::KonError::AudioDecodeFailed(format!("Task join error: {e}")))?
+}