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>

e2e/tests/journeys.spec.ts

@@ -122,9 +122,9 @@ test('the coverage figure is live, not the hardcoded fallback', async ({ page })
 test('the landing page states the real comparison limit', async ({ page }) => {
   await page.goto('/');
   const body = await page.locator('body').innerText();
-  // MAX_SCHOOLS in context/ComparisonProvider.tsx is 5. The hero used to
-  // promise three while a card lower on the same page promised five.
-  expect(body).toMatch(/five schools/i);
+  // MAX_PER_GROUP in lib/compareLogic.ts is 5 per phase. The hero once
+  // promised three while a card lower on the same page promised five.
+  expect(body).toMatch(/five primary and five secondary schools/i);
   expect(body).not.toMatch(/three schools/i);
 });