fix(seo): noindex parameterised comparisons, keep bare /compare

25,193 schools make ~317 million pairs. The bare page stays indexable as the
landing page for the head term; the parameter space goes noindex, follow so
its outbound links still count.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015mWQnpye9F299NVRCCSRvj

nextjs-app/__tests__/app/metadata.test.ts

@@ -1,6 +1,7 @@
 import { metadata as homeMetadata } from '@/app/page';
 import { metadata as rankingsMetadata } from '@/app/rankings/page';
 import { metadata as admissionsMetadata } from '@/app/admissions/page';
+import { generateMetadata as compareMetadata } from '@/app/compare/page';
 
 describe('canonical URLs', () => {
   it('the homepage canonicalises to the bare root', () => {
@@ -21,3 +22,32 @@ describe('canonical URLs', () => {
       .toBe('https://www.schoolcompare.co.uk/admissions');
   });
 });
+
+describe('/compare indexability', () => {
+  it('the bare compare page is indexable and canonical to itself', async () => {
+    // This is the landing page for the "compare schools" head term.
+    const meta = await compareMetadata({ searchParams: Promise.resolve({}) });
+    expect(meta.alternates?.canonical)
+      .toBe('https://www.schoolcompare.co.uk/compare');
+    expect(meta.robots).toBeUndefined();
+  });
+
+  it('a comparison of specific schools is noindex, follow', async () => {
+    // ~317 million pairs before triples. Indexing the parameter space would
+    // swamp everything else in the corpus.
+    const meta = await compareMetadata({
+      searchParams: Promise.resolve({ urns: '100001,100002' }),
+    });
+    expect(meta.robots).toEqual({ index: false, follow: true });
+  });
+
+  it('a parameterised comparison still canonicalises to the bare path', async () => {
+    // follow:true plus a canonical means the outbound links to each school
+    // page still pass value even though this URL is not indexed.
+    const meta = await compareMetadata({
+      searchParams: Promise.resolve({ urns: '100001,100002' }),
+    });
+    expect(meta.alternates?.canonical)
+      .toBe('https://www.schoolcompare.co.uk/compare');
+  });
+});