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/test/fakeEventSource.ts

@@ -4,15 +4,26 @@ export class FakeEventSource {
   onopen: (() => void) | null = null;
   onerror: (() => void) | null = null;
   closed = false;
+  private listeners = new Map<string, Set<(e: { data: string }) => void>>();
   constructor(public url: string) {
     FakeEventSource.instances.push(this);
   }
+  /** Dispatch to the default onmessage handler (unnamed `event: message`). */
   emit(obj: unknown) {
     this.onmessage?.({ data: JSON.stringify(obj) });
   }
-  // no-op stubs so useSessionStream's addEventListener calls don't throw
-  addEventListener(_type: string, _handler: unknown) {}
-  removeEventListener(_type: string, _handler: unknown) {}
+  /** Dispatch to handlers registered for a NAMED event (e.g. "ui_request"). */
+  emitNamed(type: string, obj: unknown) {
+    const data = JSON.stringify(obj);
+    for (const h of this.listeners.get(type) ?? []) h({ data });
+  }
+  addEventListener(type: string, handler: (e: { data: string }) => void) {
+    if (!this.listeners.has(type)) this.listeners.set(type, new Set());
+    this.listeners.get(type)!.add(handler);
+  }
+  removeEventListener(type: string, handler: (e: { data: string }) => void) {
+    this.listeners.get(type)?.delete(handler);
+  }
   close() {
     this.closed = true;
   }