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/domain/tests/background_task.rs

@@ -134,6 +134,11 @@ fn allowed_non_terminal_transitions_update_timestamp() {
         .transition(BackgroundTaskState::Running, 140)
         .unwrap();
     assert_eq!(running_again.state, BackgroundTaskState::Running);
+
+    let cancelling = running_again
+        .transition(BackgroundTaskState::Cancelling, 150)
+        .unwrap();
+    assert_eq!(cancelling.state, BackgroundTaskState::Cancelling);
 }
 
 #[test]
@@ -245,6 +250,18 @@ fn result_variants_determine_terminal_state() {
             .state,
         BackgroundTaskState::Expired
     );
+    assert_eq!(
+        running
+            .transition(BackgroundTaskState::Cancelling, 125)
+            .unwrap()
+            .complete(BackgroundTaskResult::CancelFailed {
+                finished_at_ms: 130,
+                error: "kill failed".into(),
+            })
+            .unwrap()
+            .state,
+        BackgroundTaskState::CancelFailed
+    );
 }
 
 #[test]