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/app/school/[slug]/page.tsx

@@ -4,11 +4,12 @@
  * URL format: /school/138267-school-name-here
  */
 
-import { fetchSchoolDetails, fetchSchools } from '@/lib/api';
+import { fetchSchoolDetails, fetchSchools, fetchNationalAverages } from '@/lib/api';
 import { notFound, redirect } from 'next/navigation';
 import { SchoolDetailView } from '@/components/SchoolDetailView';
 import { SecondarySchoolDetailView } from '@/components/SecondarySchoolDetailView';
 import { parseSchoolSlug, schoolUrl } from '@/lib/utils';
+import type { NationalAverages } from '@/lib/types';
 import type { Metadata } from 'next';
 
 /**
@@ -124,10 +125,18 @@ export default async function SchoolPage({ params }: SchoolPageProps) {
     notFound();
   }
 
-  // Fetch school data
+  // Fetch school data. National averages feed the England-comparison deltas
+  // across most sections; fetching them here rather than in a client effect
+  // keeps those sections server-renderable and puts the deltas in the initial
+  // HTML. They are supplementary, so they degrade to null rather than 404ing
+  // the page.
   let data;
+  let nationalAvg: NationalAverages | null = null;
   try {
-    data = await fetchSchoolDetails(urn);
+    [data, nationalAvg] = await Promise.all([
+      fetchSchoolDetails(urn),
+      fetchNationalAverages().catch(() => null),
+    ]);
   } catch (error) {
     console.error(`Failed to fetch school ${urn}:`, error);
     notFound();
@@ -193,6 +202,7 @@ export default async function SchoolPage({ params }: SchoolPageProps) {
           admissions={admissions ?? null}
           deprivation={deprivation ?? null}
           finance={finance ?? null}
+          nationalAvg={nationalAvg}
         />
       ) : (
         <SchoolDetailView
@@ -205,6 +215,7 @@ export default async function SchoolPage({ params }: SchoolPageProps) {
           admissionsHistory={admissions_history ?? []}
           deprivation={deprivation ?? null}
           finance={finance ?? null}
+          nationalAvg={nationalAvg}
         />
       )}
     </>