agent: code-atomiser-fix — cancellable Whisper inference + bounded drain + lock-over-await

Three findings chain: thread::spawn discarded the JoinHandle and gave
no cancellation route into the whisper backend; drain_inference busy-
polled with no timeout; stop_live_transcription_session held the
lifecycle AsyncMutex across the await of a non-cancellable
spawn_blocking handle. Together: a single wedged inference (ggml
deadlock, GPU stall) bricked every future start/stop until app restart.

Fix: each inference task carries an Arc<AtomicBool> abort_flag; the
flag is wired into whisper-rs::FullParams::set_abort_callback_safe so
the spawned blocking thread checks it and exits cleanly. drain_inference
is bounded by a deadline (3 x chunk_duration, min 2s); on expiry the
flag is set, the receiver is dropped, and a typed Error::InferenceTimeout
status is surfaced. Drop for InferenceTask asserts the abort flag so
any '?'-propagation or panic unwind closes the cancellation route
without relying on the explicit drain path. stop_live_transcription_session
restructured so the lifecycle guard is dropped BEFORE the JoinHandle
is awaited; start_live_transcription_session releases the guard
explicitly after installing the RunningLiveSession.

Unit test added: dropping_inference_task_sets_abort_flag covers the
Race-A regression directly. A real Race-B drain-timeout test would
require a wedged whisper-rs, which is hard to fixture; that path is
covered by the SAFETY comment and cargo build + manual smoke.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/transcription/src/local_engine.rs

@@ -1,5 +1,6 @@
 use std::path::Path;
-use std::sync::Mutex;
+use std::sync::atomic::AtomicBool;
+use std::sync::{Arc, Mutex};
 use std::time::Instant;
 
 use transcribe_rs::{SpeechModel, TranscribeOptions, TranscriptionResult};
@@ -155,6 +156,35 @@ impl LocalEngine {
             inference_ms,
         })
     }
+
+    /// Cancellable variant of `transcribe_sync`. Pipes `abort_flag`
+    /// through to the backend so the live-session drain timeout can
+    /// break a wedged whisper-rs decode out of its loop. Backends that
+    /// can't honour the flag (Parakeet) fall through to their plain
+    /// `transcribe_sync` via the trait's default implementation.
+    pub fn transcribe_sync_with_abort(
+        &self,
+        audio: &AudioSamples,
+        options: &TranscriptionOptions,
+        abort_flag: Arc<AtomicBool>,
+    ) -> Result<TimedTranscript> {
+        let mut guard = self.engine.lock().unwrap_or_else(|e| e.into_inner());
+        let backend = guard.as_mut().ok_or(Error::EngineNotLoaded)?;
+
+        let start = Instant::now();
+        let segments =
+            backend.transcribe_sync_with_abort(audio.samples(), options, abort_flag)?;
+        let inference_ms = start.elapsed().as_millis() as u64;
+
+        Ok(TimedTranscript {
+            transcript: Transcript::new(
+                segments,
+                options.language.clone().unwrap_or_else(|| "en".to_string()),
+                audio.duration_secs(),
+            ),
+            inference_ms,
+        })
+    }
 }
 
 /// Thin wrapper over `ParakeetModel` that overrides `transcribe_raw` to