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/src/background_task.rs

@@ -28,12 +28,16 @@ pub enum BackgroundTaskState {
     Running,
     /// Suspended while waiting for an external event.
     Waiting,
+    /// Cancellation has been accepted and the runner is stopping the resource.
+    Cancelling,
     /// Finished successfully.
     Completed,
     /// Finished with an error.
     Failed,
     /// Cancelled before natural completion.
     Cancelled,
+    /// Cancellation was requested but the runner could not stop the resource.
+    CancelFailed,
     /// Expired after its deadline.
     Expired,
 }
@@ -44,7 +48,7 @@ impl BackgroundTaskState {
     pub const fn is_terminal(self) -> bool {
         matches!(
             self,
-            Self::Completed | Self::Failed | Self::Cancelled | Self::Expired
+            Self::Completed | Self::Failed | Self::Cancelled | Self::CancelFailed | Self::Expired
         )
     }
 }
@@ -342,6 +346,13 @@ pub enum BackgroundTaskResult {
         /// Cancellation reason.
         reason: String,
     },
+    /// Cancellation failed.
+    CancelFailed {
+        /// Completion timestamp, epoch milliseconds.
+        finished_at_ms: u64,
+        /// Human-readable error.
+        error: String,
+    },
     /// Expired completion.
     Expired {
         /// Completion timestamp, epoch milliseconds.
@@ -375,6 +386,7 @@ impl BackgroundTaskResult {
             Self::Success { finished_at_ms, .. }
             | Self::Failure { finished_at_ms, .. }
             | Self::Cancelled { finished_at_ms, .. }
+            | Self::CancelFailed { finished_at_ms, .. }
             | Self::Expired { finished_at_ms, .. } => *finished_at_ms,
         }
     }
@@ -408,6 +420,9 @@ impl BackgroundTaskResult {
             Self::Cancelled { reason, .. } | Self::Expired { reason, .. } => {
                 validate_text("reason", reason)?;
             }
+            Self::CancelFailed { error, .. } => {
+                validate_text("error", error)?;
+            }
         }
         Ok(())
     }
@@ -417,6 +432,7 @@ impl BackgroundTaskResult {
             Self::Success { .. } => BackgroundTaskState::Completed,
             Self::Failure { .. } => BackgroundTaskState::Failed,
             Self::Cancelled { .. } => BackgroundTaskState::Cancelled,
+            Self::CancelFailed { .. } => BackgroundTaskState::CancelFailed,
             Self::Expired { .. } => BackgroundTaskState::Expired,
         }
     }
@@ -459,13 +475,22 @@ pub enum BackgroundTaskError {
 }
 
 fn is_transition_allowed(from: BackgroundTaskState, to: BackgroundTaskState) -> bool {
-    use BackgroundTaskState::{Cancelled, Completed, Expired, Failed, Queued, Running, Waiting};
+    use BackgroundTaskState::{
+        CancelFailed, Cancelled, Cancelling, Completed, Expired, Failed, Queued, Running, Waiting,
+    };
 
     matches!(
         (from, to),
-        (Queued, Running | Cancelled | Expired)
-            | (Running, Waiting | Completed | Failed | Cancelled | Expired)
-            | (Waiting, Running | Completed | Failed | Cancelled | Expired)
+        (
+            Queued,
+            Running | Cancelling | Cancelled | CancelFailed | Expired
+        ) | (
+            Running,
+            Waiting | Cancelling | Completed | Failed | Cancelled | CancelFailed | Expired
+        ) | (
+            Waiting,
+            Running | Cancelling | Completed | Failed | Cancelled | CancelFailed | Expired
+        ) | (Cancelling, Cancelled | CancelFailed | Expired)
     )
 }