fix(backend): track Pi turn lifecycle

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

@@ -94,6 +94,7 @@ export class PiProcessManager {
     const rpc = new RpcClient(child);
     const bridge = new SessionBridge(rpc);
     const rt: SessionRuntime = { rpc, bridge, child };
+    bridge.beginTurn();
     this.runtimes.set(sessionId, rt);
     // Identity-checked: a stale child's exit must not evict a newer runtime.
     // Expected teardowns (teardown()/respawn) delete the runtime from the map BEFORE the
@@ -136,6 +137,7 @@ export class PiProcessManager {
     const message = o.mode === "resume"
       ? `/riprendi-sessione ${sessionId}`
       : `/nuova-domanda ${JSON.stringify(o.question ?? "")}`;
+    rt.bridge.beginTurn();
     rt.rpc.send({ type: "prompt", message });
   }