fix(audio): log dropped capture errors instead of silently discarding

The cpal stream-error closure used `let _ = err_tx.try_send(...)` against a
bounded sync_channel(16). If the live session's listener stalled or the
frontend disconnected, runtime stream errors were silently dropped — the
diagnostic bundle showed nothing for a session that mysteriously stopped
working.

- Bump the error channel capacity 16 → 32 (matches AUDIO_CHANNEL_CAPACITY).
- On try_send failure, log to stderr with the device name + a per-session
  drop counter so the symptom is visible in the diagnostic bundle even
  when the typed event never reached the frontend.
- Plumb a new `dropped_errors: Arc<AtomicU64>` through `build_input_stream`
  alongside the existing `dropped_chunks`, mirroring the same pattern.

(kon-audio doesn't build in the audit sandbox: it links against ALSA
which the sandbox lacks. CI cross-platform compiles it.)

https://claude.ai/code/session_0189xUb6ie6t9qHkzatGZ9Rb

crates/audio/src/capture.rs

@@ -370,11 +370,15 @@ fn open_and_validate(
     let (tx, rx) = mpsc::sync_channel::<AudioChunk>(AUDIO_CHANNEL_CAPACITY);
     let requeue_tx = tx.clone();
     let dropped_chunks = Arc::new(AtomicU64::new(0));
-    // Bounded channel for runtime stream errors. Capacity 16 = plenty for
+    // Bounded channel for runtime stream errors. Capacity 32 = plenty for
-    // the rare error case; if it ever fills, we drop newer errors silently
+    // the rare error case; if it ever fills, drops are reported via stderr
-    // because they would be redundant noise in a stream that is already
+    // and counted in `dropped_errors` so the symptom is visible in the
-    // failing. (Codex review 2026/04/17 M2)
+    // diagnostic bundle even when the listener has gone away. Errors
-    let (err_tx, err_rx) = mpsc::sync_channel::<CaptureRuntimeError>(16);
+    // beyond the cap are by definition redundant noise in a stream that
+    // is already failing. (Codex review 2026/04/17 M2; capacity bump and
+    // drop logging added 2026/04/25 audit pass.)
+    let (err_tx, err_rx) = mpsc::sync_channel::<CaptureRuntimeError>(32);
+    let dropped_errors = Arc::new(AtomicU64::new(0));
 
     let stream = match format {
         SampleFormat::F32 => build_input_stream::<f32>(
@@ -385,6 +389,7 @@ fn open_and_validate(
             tx,
             dropped_chunks.clone(),
             err_tx.clone(),
+            dropped_errors.clone(),
             name.to_string(),
         ),
         SampleFormat::I16 => build_input_stream::<i16>(
@@ -395,6 +400,7 @@ fn open_and_validate(
             tx,
             dropped_chunks.clone(),
             err_tx.clone(),
+            dropped_errors.clone(),
             name.to_string(),
         ),
         SampleFormat::U16 => build_input_stream::<u16>(
@@ -405,6 +411,7 @@ fn open_and_validate(
             tx,
             dropped_chunks.clone(),
             err_tx.clone(),
+            dropped_errors.clone(),
             name.to_string(),
         ),
         other => {
@@ -503,6 +510,7 @@ fn build_input_stream<T>(
     tx: mpsc::SyncSender<AudioChunk>,
     dropped_chunks: Arc<AtomicU64>,
     err_tx: mpsc::SyncSender<CaptureRuntimeError>,
+    dropped_errors: Arc<AtomicU64>,
     device_name: String,
 ) -> std::result::Result<cpal::Stream, cpal::BuildStreamError>
 where
@@ -532,10 +540,24 @@ where
             // frontend can show a toast. Also keep the eprintln for ops
             // logs. (Codex review 2026/04/17 M2)
             eprintln!("[kon-audio] capture error: {err}");
-            let _ = err_tx.try_send(CaptureRuntimeError {
+            if err_tx
+                .try_send(CaptureRuntimeError {
                     device_name: err_device_name.clone(),
                     message: err.to_string(),
-            });
+                })
+                .is_err()
+            {
+                // Channel full — listener has stalled or detached. Note
+                // it in stderr and the dropped-errors counter so the
+                // diagnostic bundle still shows the symptom even if the
+                // frontend never received the typed event.
+                let prior = dropped_errors.fetch_add(1, Ordering::Relaxed);
+                eprintln!(
+                    "[kon-audio] capture error channel full; dropped error #{} for device '{}'",
+                    prior + 1,
+                    err_device_name,
+                );
+            }
         },
         None,
     )