fix(backend): idempotent spawnFor + identity-checked exit + unified SSE re-emit shape

- spawnFor now tears down any existing runtime for the same session id before
  the cap check, so resume/respawn neither leaks the old child nor falsely hits
  maxPiProcesses
- exit handler is identity-checked (captures rt) so a stale child's late exit
  cannot evict a newer runtime
- SSE pending re-emit now sends the full ClientEvent shape
  { type: "ui_request", ui_request } to match hub.publish live events
- tests: same-id respawn replaces runtime (count 1); old child exit does not
  evict new runtime; sse-hub re-emit asserts unified shape

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -34,6 +34,33 @@ test("l'exit del child rimuove il runtime dalla mappa (exit handler)", async ()
   expect(mgr.get("exit-test")).toBeUndefined();
 });
 
+test("spawnFor sullo STESSO id uccide il vecchio child e sostituisce il runtime (count resta 1)", async () => {
+  const cfg = loadConfig({ THT_HARNESS_DIR: "../harness" });
+  const mgr = new PiProcessManager(cfg, { spawnFn: () => spawn("node", [FAKE, SCRIPT]) as any });
+  const first = await mgr.spawnFor("dup-id", {});
+  expect(mgr.count()).toBe(1);
+  const firstExited = new Promise<void>((res) => first.child.on("exit", () => res()));
+  const second = await mgr.spawnFor("dup-id", {});
+  await firstExited; // the old child was killed by the idempotent respawn
+  expect(mgr.count()).toBe(1);
+  expect(mgr.get("dup-id")).toBe(second);
+  expect(second).not.toBe(first);
+  mgr.teardown("dup-id");
+});
+
+test("l'exit del VECCHIO child non elimina il nuovo runtime (exit identity-checked)", async () => {
+  const cfg = loadConfig({ THT_HARNESS_DIR: "../harness" });
+  const mgr = new PiProcessManager(cfg, { spawnFn: () => spawn("node", [FAKE, SCRIPT]) as any });
+  const first = await mgr.spawnFor("respawn-id", {});
+  const second = await mgr.spawnFor("respawn-id", {});
+  // The old child's exit handler fires after the respawn; it must NOT evict `second`.
+  await new Promise((res) => setImmediate(res));
+  expect(mgr.get("respawn-id")).toBe(second);
+  expect(mgr.count()).toBe(1);
+  void first;
+  mgr.teardown("respawn-id");
+});
+
 test("oltre maxPiProcesses solleva errore", async () => {
   const cfg = { ...loadConfig({}), maxPiProcesses: 1 };
   const mgr = new PiProcessManager(cfg, { spawnFn: () => spawn("node", [FAKE, SCRIPT]) as any });