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/lib/compareLogic.ts

@@ -289,3 +289,47 @@ export function latestValues(
     return null;
   });
 }
+
+// ---------------------------------------------------------------------------
+// Basket limits — five per comparison group, not five overall
+// ---------------------------------------------------------------------------
+
+export type CompareGroup = 'primary' | 'secondary';
+
+export const MAX_PER_GROUP = 5;
+
+/**
+ * Which phase tab(s) of the compare page a school can land in. The page
+ * sorts by the data a school actually has, which isn't known when it's added,
+ * so anything that isn't plainly one phase (all-through, special schools
+ * with phase "Not applicable", unknown) counts against BOTH groups. That
+ * keeps every tab at MAX_PER_GROUP or fewer, which the five-slot chart
+ * palette and point styles rely on.
+ */
+export function compareGroups(phase?: string | null): CompareGroup[] {
+  const p = (phase ?? '').toLowerCase();
+  // "Middle deemed secondary" / "Middle deemed primary" match here too.
+  if (p.includes('secondary')) return ['secondary'];
+  if (p.includes('primary')) return ['primary'];
+  return ['primary', 'secondary'];
+}
+
+/** The group that has no room for `school`, or null if it can be added. */
+export function fullGroupFor(
+  basket: ReadonlyArray<{ phase?: string | null }>,
+  school: { phase?: string | null },
+): CompareGroup | null {
+  for (const group of compareGroups(school.phase)) {
+    const used = basket.filter((s) => compareGroups(s.phase).includes(group)).length;
+    if (used >= MAX_PER_GROUP) return group;
+  }
+  return null;
+}
+
+/** Keep schools in order, dropping any that would overflow a group. */
+export function fitToGroupLimits<T extends { phase?: string | null }>(schools: T[]): T[] {
+  return schools.reduce<T[]>(
+    (kept, school) => (fullGroupFor(kept, school) ? kept : [...kept, school]),
+    [],
+  );
+}