fix: serialize session lifecycle transitions

backend/src/pi/pi-process-manager.ts

@@ -174,9 +174,16 @@ export class PiProcessManager {
 
   teardown(id: string): void {
     const rt = this.runtimes.get(id);
-    if (rt) {
-      rt.child.kill();
-      this.runtimes.delete(id);
-    }
+    if (rt) this.teardownIfCurrent(id, rt);
+  }
+
+  /** Remove only the runtime identity the caller observed. */
+  teardownIfCurrent(id: string, expected: SessionRuntime): boolean {
+    if (this.runtimes.get(id) !== expected) return false;
+    // Delete before signalling the child so its asynchronous exit cannot be mistaken for a
+    // crash, and so a replacement installed by a later lifecycle operation is never targeted.
+    this.runtimes.delete(id);
+    expected.child.kill();
+    return true;
   }
 }