test(frontend): unit tests exercise named SSE events (fidelity) + cleanup

FakeEventSource.addEventListener was a no-op, so emit() only drove onmessage.
Production relies on addEventListener for the backend's NAMED events
(event: ui_request), so the unit tests could pass while prod silently broke.

- fakeEventSource: store named handlers in a Map<string,Set>; add emitNamed()
  that dispatches to them; keep emit() for the unnamed/default onmessage path
- useSessionStream.test: ui_request now driven via emitNamed (production path);
  add a separate test for the unnamed text_delta path via plain emit
- f1-loop.test: widget emission switched to emitNamed("ui_request", ...)
- verified: tests FAIL if addEventListener wiring is removed from
  useSessionStream (then restored)
- cleanup: fake-pi.mjs drops unused execFileSync import, uses static spawnSync

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/stream/useSessionStream.test.tsx

@@ -10,16 +10,25 @@ beforeEach(() => {
   useSessionStore.getState().resetSession();
 });
 
-test("opens an EventSource for the session and feeds events to the store", () => {
+test("opens an EventSource and feeds NAMED events to the store", () => {
   renderHook(() => useSessionStream("s1"));
   const es = FakeEventSource.instances[0];
   expect(es.url).toContain("/sessions/s1/events");
+  // Backend sends `event: ui_request` (named) — drive the addEventListener path
+  // that production relies on, not the unnamed onmessage fallback.
   act(() =>
-    es.emit({ type: "ui_request", ui_request: { id: "u1", widget: "select" } })
+    es.emitNamed("ui_request", { type: "ui_request", ui_request: { id: "u1", widget: "select" } })
   );
   expect(useSessionStore.getState().pendingWidget?.id).toBe("u1");
 });
 
+test("also feeds UNNAMED (default message) events to the store", () => {
+  renderHook(() => useSessionStream("s1"));
+  const es = FakeEventSource.instances[0];
+  act(() => es.emit({ type: "text_delta", text: "hello" }));
+  expect(useSessionStore.getState().transcript.at(-1)?.text).toBe("hello");
+});
+
 test("closes the stream on unmount", () => {
   const { unmount } = renderHook(() => useSessionStream("s1"));
   const es = FakeEventSource.instances[0];