fix(resume): recover cleanly after Pi exits

backend/src/bridge/session-bridge.ts

@@ -33,7 +33,7 @@ export class SessionBridge {
         m.message?.role === "assistant" &&
         m.message?.stopReason === "error"
       ) {
-        this.state = "failed";
+        this.markFailed();
         this.fan({
           type: "info",
           level: "error",
@@ -69,6 +69,9 @@ export class SessionBridge {
 
   beginTurn(): void { this.state = "running"; }
 
+  /** Record a backend-detected failure; callers own any sanitized client message. */
+  markFailed(): void { this.state = "failed"; }
+
   onClientEvent(cb: (e: ClientEvent) => void): void { this.cbs.add(cb); }
 
   /** Eventi generati dal backend stesso (es. exit inatteso del child Pi), non da Pi. */