refactor(detail): fetch national averages on the server

Both detail views fetched /api/national-averages in a useEffect, so the
England-comparison deltas popped in after hydration and every section that
uses them was pinned to the client. The page now fetches it in parallel with
the school details (backend-cached 1h, degrades to null) and passes it down.

Removes one client round-trip per detail page and unblocks the section
extraction.

Characterization tests pass unmodified; only the render helper changed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

nextjs-app/__tests__/support/renderSchoolDetail.tsx

@@ -6,9 +6,8 @@
  * suite may change — the characterization assertions passing unmodified across
  * that rewrite is the proof that behaviour was preserved.
  *
- * National averages are currently fetched client-side via useEffect, so this
- * helper stubs global.fetch. Once they arrive as a server-supplied prop the
- * stub goes away; the tests use findBy* queries so they pass either way.
+ * National averages now arrive as a server-supplied prop rather than a client
+ * fetch, so no fetch stub is needed.
  */
 
 import { render } from '@testing-library/react';
@@ -24,20 +23,16 @@ function withProviders(ui: ReactNode) {
   return <ComparisonProvider>{ui}</ComparisonProvider>;
 }
 
-function stubNationalAveragesFetch() {
-  global.fetch = jest.fn((url: any) =>
-    String(url).includes('national-averages')
-      ? Promise.resolve({ ok: true, json: () => Promise.resolve(nationalAveragesFixture) })
-      : Promise.resolve({ ok: false, json: () => Promise.resolve({}) }),
-  ) as unknown as typeof fetch;
-}
-
 export function renderSchoolDetail(fixture: any) {
-  stubNationalAveragesFetch();
-  return render(withProviders(<SchoolDetailView {...fixture} />));
+  return render(
+    withProviders(<SchoolDetailView {...fixture} nationalAvg={nationalAveragesFixture} />),
+  );
 }
 
 export function renderSecondarySchoolDetail(fixture: any) {
-  stubNationalAveragesFetch();
-  return render(withProviders(<SecondarySchoolDetailView {...fixture} />));
+  return render(
+    withProviders(
+      <SecondarySchoolDetailView {...fixture} nationalAvg={nationalAveragesFixture} />,
+    ),
+  );
 }