test(explore): add the factory-closure fixture and its selection probe (CG-27)

A file whose top-level symbol spans almost all of it — createFoo() returning
an object of closures — is how Svelte 5 rune stores, React custom-hook modules,
IIFE module-pattern JS and Zustand's create((set,get)=>({…})) are all written.
probe-factory-closure.mjs measures what such a file DELIVERS from within: which
inner symbols' definitions reach the agent, not how many bytes did.

__tests__/fixtures/factory-closure-ts/src/ui/panel.ts

@@ -0,0 +1,43 @@
+import type { DashboardStore } from '../stores/dashboard-store';
+import type { FilterSpec } from '../stores/types';
+import { medianOf } from '../lib/metrics';
+
+/** The dashboard panel — the only consumer of the store's closures. */
+export function mountPanel(store: DashboardStore, dashboardId: string) {
+  let disposed = false;
+
+  const unsubscribe = store.subscribe((state) => {
+    if (disposed) return;
+    render(state.widgets.length, state.sampleCount, state.loading);
+  });
+
+  async function boot(): Promise<void> {
+    await store.loadWidgets(dashboardId);
+    await store.refreshMetrics();
+    store.reconcileLayout();
+  }
+
+  function search(text: string): void {
+    const specs: FilterSpec[] = text.trim().length === 0
+      ? []
+      : [{ field: 'title', op: 'contains', value: text.trim() }];
+    store.applyFilter(specs);
+  }
+
+  function download(): string {
+    return store.exportCsv();
+  }
+
+  function render(widgetCount: number, sampleCount: number, loading: boolean): void {
+    void widgetCount;
+    void sampleCount;
+    void loading;
+  }
+
+  function dispose(): void {
+    disposed = true;
+    unsubscribe();
+  }
+
+  return { boot, search, download, dispose, median: medianOf };
+}