audio: Day 2 — Codex follow-up hardening (channel disconnect, spawn_blocking, fallback silence guard, requeue counting, runtime error propagation)

Closes the 6 Codex review findings on the Day 1 mic-capture work
(commits 96980c7 + 41db162). Detail in
output/reports/kon-codex-mic-capture-followup-2026-04-17.md (CORBEL
workspace).

src-tauri/src/commands/audio.rs:
- D1: Sending an explicit stop signal before dropping stop_tx, so the
  accumulator task wakes up immediately rather than waiting for the
  Disconnected detection added below.
- D2: Wrapping MicrophoneCapture::start() in tokio::task::spawn_blocking.
  start() can spend up to 350ms × N_devices × 2 passes; running it on
  the async runtime froze other Tauri commands.
- M3: Match on rx.try_recv() error variants. Empty -> sleep + continue.
  Disconnected -> exit accumulator task immediately. Previous behaviour
  was an infinite loop after the capture stream died.

crates/audio/src/capture.rs:
- D3: Added DEAD_SILENCE_FLOOR (1e-7) gate that applies even in the
  fallback pass. PulseAudio/PipeWire can stream zero-valued bytes from
  a borked device; that is worse than failing fast.
- M1: Validation requeue (`for chunk in collected { try_send }`) now
  counts drops in the same dropped_chunks counter.
- M2: New CaptureRuntimeError type. The cpal stream error callback now
  forwards errors via a separate sync_channel (capacity 16) that the
  live session can drain via take_error_rx() and surface as toasts.
  Re-exported from kon_audio crate root.

cargo check -p kon-audio passes clean.

NOT YET DONE (M4): JACK-specific monitor name patterns. Defers until
testing on a JACK host. Current is_monitor_name() may miss JACK
conventions.

Wiring the new error_rx into the live session and surfacing as toasts
lands with the Day 3 error-toast system commit.

crates/audio/src/lib.rs

@@ -6,7 +6,7 @@ pub mod streaming_resample;
 pub mod vad;
 pub mod wav;
 
-pub use capture::{AudioChunk, DeviceInfo, MicrophoneCapture};
+pub use capture::{AudioChunk, CaptureRuntimeError, DeviceInfo, MicrophoneCapture};
 pub use concurrency::decode_and_resample;
 pub use decode::decode_audio_file;
 pub use resample::resample_to_16khz;