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/shell/f1-loop.test.tsx

@@ -41,9 +41,10 @@ test("F1: create session -> widget via SSE -> respond -> POST /response", async
   // Wait for the session to be created and SSE to connect
   await waitFor(() => expect(FakeEventSource.instances).toHaveLength(1));
 
-  // Simulate the backend emitting the F1 widget
+  // Simulate the backend emitting the F1 widget via a NAMED SSE event
+  // (`event: ui_request`) — the same addEventListener path production uses.
   act(() =>
-    FakeEventSource.instances[0].emit({
+    FakeEventSource.instances[0].emitNamed("ui_request", {
       type: "ui_request",
       ui_request: {
         id: "u1",