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/capture.rs

@@ -0,0 +1,75 @@
+use std::sync::mpsc;
+
+use cpal::traits::{DeviceTrait, HostTrait, StreamTrait};
+
+use kon_core::error::{KonError, Result};
+
+/// A chunk of captured audio from the microphone.
+pub struct AudioChunk {
+    pub samples: Vec<f32>,
+    pub sample_rate: u32,
+    pub channels: u16,
+}
+
+/// Manages microphone capture via cpal.
+/// Call `start()` to begin capturing, which returns a receiver for audio chunks.
+/// Call `stop()` to end the stream.
+pub struct MicrophoneCapture {
+    stream: Option<cpal::Stream>,
+}
+
+impl MicrophoneCapture {
+    /// Start capturing audio from the default input device.
+    /// Returns a receiver that yields AudioChunks as they arrive.
+    pub fn start() -> Result<(Self, mpsc::Receiver<AudioChunk>)> {
+        let host = cpal::default_host();
+        let device = host.default_input_device().ok_or_else(|| {
+            KonError::AudioCaptureFailed("No input device found".into())
+        })?;
+
+        let config = device.default_input_config().map_err(|e| {
+            KonError::AudioCaptureFailed(format!("No input config: {e}"))
+        })?;
+
+        let sample_rate = config.sample_rate();
+        let channels = config.channels() as u16;
+
+        let (tx, rx) = mpsc::channel::<AudioChunk>();
+
+        let stream = device
+            .build_input_stream(
+                &config.into(),
+                move |data: &[f32], _info: &cpal::InputCallbackInfo| {
+                    let _ = tx.send(AudioChunk {
+                        samples: data.to_vec(),
+                        sample_rate,
+                        channels,
+                    });
+                },
+                |err| eprintln!("audio capture error: {err}"),
+                None,
+            )
+            .map_err(|e| {
+                KonError::AudioCaptureFailed(format!("Build stream failed: {e}"))
+            })?;
+
+        stream.play().map_err(|e| {
+            KonError::AudioCaptureFailed(format!("Stream play failed: {e}"))
+        })?;
+
+        Ok((Self { stream: Some(stream) }, rx))
+    }
+
+    /// Stop capturing audio.
+    pub fn stop(&mut self) {
+        if let Some(stream) = self.stream.take() {
+            let _ = stream.pause();
+        }
+    }
+}
+
+impl Drop for MicrophoneCapture {
+    fn drop(&mut self) {
+        self.stop();
+    }
+}