fix(runtime): cancel de background task immédiat et fiable depuis le workstate (#199, QA verte)

Transforme l'annulation d'une background task en contrat immédiat, observable et non-best-effort:

- domain/background_task: états Cancelling/Cancelled/CancelFailed et transitions idempotentes; un kill PTY qui échoue est surfacé en CancelFailed plutôt qu'en succès silencieux.

- infrastructure/runner + store: cancel effectif de la task ciblée, reconcile marque Cancelling même sans live handle, et départage CancelFailed des tâches pending au boot.

- application/background + workstate + orchestrator/wake: chemin de cancel initié depuis le workstate, projection synchronisée de l'état (pas d'attente d'un cycle ambigu).

- frontend: projection workstate de l'état cancelled/cancelling/cancelFailed (cancellingRow/cancelFailedRow/cancelButtons), normalization et services.

Tests QA verts: background_task_cancel, background_task_runner cancel_wins_even_if_output_is_still_available, background_task_store reconcile_marks_cancelling_without_live_handle_cancel_failed_and_pending, workstate.test.tsx.

crates/infrastructure/src/background_task/runner.rs

@@ -139,11 +139,14 @@ impl CommandBackgroundRunner {
             .filter(|s| !s.is_empty());
 
         // Reap the real exit status (works for a natural exit and a forced kill).
-        let exit_code = pty
-            .kill(&control.pty_handle)
-            .await
-            .ok()
-            .and_then(|status| status.code);
+        let exit_code = match outcome {
+            WaitOutcome::Cancelled => None,
+            WaitOutcome::Expired | WaitOutcome::Exited => pty
+                .kill(&control.pty_handle)
+                .await
+                .ok()
+                .and_then(|status| status.code),
+        };
 
         let finished_at_ms = u64::try_from(clock.now_millis().max(0)).unwrap_or(0);
         let result = match outcome {
@@ -281,8 +284,12 @@ impl BackgroundTaskRunner for CommandBackgroundRunner {
             .cloned();
         // Unknown task ⇒ already terminal or never ours: cancellation is idempotent.
         if let Some(control) = control {
+            self.pty
+                .kill(&control.pty_handle)
+                .await
+                .map_err(|e| BackgroundTaskPortError::Runner(format!("pty kill failed: {e}")))?;
             control.cancelled.store(true, Ordering::SeqCst);
-            control.cancel.notify_one();
+            control.cancel.notify_waiters();
         }
         Ok(())
     }