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/sse-hub.test.ts

@@ -4,7 +4,7 @@ import { SseHub } from "../src/sse/sse-hub.js";
 test("re-emette il widget pendente alla sottoscrizione", () => {
   const hub = new SseHub(); const sent: any[] = [];
   hub.subscribe("s1", (ev, data) => sent.push({ ev, data }), { id: "u1", widget: "select" });
-  expect(sent[0]).toEqual({ ev: "ui_request", data: { ui_request: { id: "u1", widget: "select" } } });
+  expect(sent[0]).toEqual({ ev: "ui_request", data: { type: "ui_request", ui_request: { id: "u1", widget: "select" } } });
 });
 
 test("publish raggiunge i subscriber e unsubscribe li stacca", () => {