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/backend/src/lib.rs

@@ -1357,6 +1357,7 @@ fn background_task_tail_bytes(task: &BackgroundTask) -> Vec<u8> {
             .into_bytes(),
         Some(BackgroundTaskResult::Cancelled { reason, .. })
         | Some(BackgroundTaskResult::Expired { reason, .. }) => reason.clone().into_bytes(),
+        Some(BackgroundTaskResult::CancelFailed { error, .. }) => error.clone().into_bytes(),
         None => Vec::new(),
     }
 }
@@ -2452,10 +2453,14 @@ impl BackendCore {
             )
             .with_events(Arc::clone(&events_port)),
         );
-        let cancel_background_task = Arc::new(CancelBackgroundTask::new(
-            Arc::clone(&background_tasks_port),
-            Arc::clone(&background_runner_port),
-        ));
+        let cancel_background_task = Arc::new(
+            CancelBackgroundTask::new(
+                Arc::clone(&background_tasks_port),
+                Arc::clone(&background_runner_port),
+                Arc::clone(&clock) as Arc<dyn Clock>,
+            )
+            .with_events(Arc::clone(&events_port)),
+        );
         let retry_background_task = Arc::new(RetryBackgroundTask::new(
             Arc::clone(&background_tasks_port),
             Arc::clone(&background_runner) as Arc<dyn BackgroundCommandArchive>,