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/app/(frontend)/compare/page.tsx

@@ -33,9 +33,9 @@ export async function generateMetadata(
     // Deliberately not the homepage's phrase. Two pages chasing "compare
     // schools" is how a site competes with itself; this one takes the tool
     // phrasing instead.
-    title: 'School Comparison Tool: Up to Five Schools at Once | schoolcompare',
+    title: 'School Comparison Tool: Primary and Secondary Schools | schoolcompare',
     description:
-      'Put up to five English schools in one table: SATs and GCSE results against the England average, Ofsted grades, and the distance places were offered.',
+      'Put up to five primary and five secondary schools in England in one table: SATs and GCSE results against the England average, Ofsted grades, and the distance places were offered.',
     keywords:
       'school comparison, compare schools, Ofsted comparison, school admissions, KS2 comparison, primary school performance',
     alternates: { canonical: absoluteUrl('/compare') },