feat(compare): limit the basket to five per phase, not five overall

A parent choosing a primary and a secondary school at once hit the old
cap of five total. The basket now holds up to five primary and five
secondary schools (ten in all), matching the compare page's phase tabs.

Schools that could land in either tab (all-through, special schools with
phase "Not applicable", unknown phase) count against both groups, so no
tab ever exceeds the five-slot chart palette and point styles.

- lib/compareLogic: compareGroups, fullGroupFor, fitToGroupLimits
- search modal disables only the full group and says which one
- rankings rows carry the phase of the tab they are ranked under
- shared ?urns= links are trimmed per group
- copy: compare metadata, homepage value prop, How it works card now
  name primary and secondary schools (also better for search intent)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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

@@ -123,7 +123,7 @@ describe('C1 snippet copy', () => {
 
   it('no C1 page claims a school count that will drift', () => {
     // The corpus moves with every data refresh; this repo has already shipped
-    // one copy bug of that kind ("three schools" against MAX_SCHOOLS = 5).
+    // one copy bug of that kind ("three schools" against a limit of five).
     for (const [, meta] of pages) {
       expect(meta.description as string).not.toMatch(/\b\d{2},\d{3}\b|\b\d{2},000\b/);
     }

nextjs-app/__tests__/lib/compareLogic.test.ts

@@ -6,8 +6,12 @@
  */
 
 import {
+  MAX_PER_GROUP,
   OFSTED_LEGACY_GRADES,
   admissionsForPhase,
+  compareGroups,
+  fitToGroupLimits,
+  fullGroupFor,
   ofstedDisplay,
   progressBand,
   rcAreaLabel,
@@ -313,3 +317,47 @@ describe('latestValues', () => {
     expect(latestValues(data, [1], 'nope')).toEqual([null]);
   });
 });
+
+describe('basket limits per comparison group', () => {
+  const school = (phase: string | null, urn = 0) => ({ urn, phase });
+  const many = (phase: string | null, n: number, from = 0) =>
+    Array.from({ length: n }, (_, i) => school(phase, from + i));
+
+  it('files each phase under its compare tab', () => {
+    expect(compareGroups('Primary')).toEqual(['primary']);
+    expect(compareGroups('Middle deemed primary')).toEqual(['primary']);
+    expect(compareGroups('Secondary')).toEqual(['secondary']);
+    expect(compareGroups('Middle deemed secondary')).toEqual(['secondary']);
+  });
+
+  it('counts schools that could land in either tab against both', () => {
+    expect(compareGroups('All-through')).toEqual(['primary', 'secondary']);
+    expect(compareGroups('Not applicable')).toEqual(['primary', 'secondary']);
+    expect(compareGroups(null)).toEqual(['primary', 'secondary']);
+  });
+
+  it('a full primary group still admits secondary schools', () => {
+    const basket = many('Primary', MAX_PER_GROUP);
+    expect(fullGroupFor(basket, school('Primary'))).toBe('primary');
+    expect(fullGroupFor(basket, school('Secondary'))).toBeNull();
+    expect(fullGroupFor(basket, school('All-through'))).toBe('primary');
+  });
+
+  it('allows ten schools in total, five per group', () => {
+    const basket = [...many('Primary', 5), ...many('Secondary', 5, 5)];
+    expect(fullGroupFor(basket, school('Primary'))).toBe('primary');
+    expect(fullGroupFor(basket, school('Secondary'))).toBe('secondary');
+  });
+
+  it('an all-through school uses a slot in both groups', () => {
+    const basket = [school('All-through'), ...many('Primary', 4, 1)];
+    expect(fullGroupFor(basket, school('Primary'))).toBe('primary');
+    expect(fullGroupFor(basket, school('Secondary'))).toBeNull();
+  });
+
+  it('trims an oversized shared link per group, keeping order', () => {
+    const urls = [...many('Primary', 7), ...many('Secondary', 3, 7)];
+    const kept = fitToGroupLimits(urls).map((s) => s.urn);
+    expect(kept).toEqual([0, 1, 2, 3, 4, 7, 8, 9]);
+  });
+});