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/components/HowItWorksSection.tsx

@@ -143,7 +143,7 @@ export function HowItWorksSection() {
                   <span className={styles.compareRowVal}>{b}</span>
                 </div>
               ))}
-              <div className={styles.compareFoot}>+ compare up to 5 schools</div>
+              <div className={styles.compareFoot}>+ up to 5 primary &amp; 5 secondary</div>
             </div>
           </div>
           <div className={styles.hiwCardBody}>
@@ -152,7 +152,7 @@ export function HowItWorksSection() {
             {/* One verb for one feature. The site previously called this
                 "compare" in the nav, "shortlist" in the footer and "pin" here,
                 which reads as three separate things it does not have. */}
-            <p className={styles.hiwDesc}>Compare up to five schools and every metric aligns in the same columns. It works for primary and secondary alike.</p>
+            <p className={styles.hiwDesc}>Compare primary and secondary school performance side by side: KS2 SATs results for primary schools, GCSE Attainment 8 for secondary schools, and Ofsted ratings for both, lined up in one table so you can see where each school is stronger.</p>
           </div>
         </div>
       </div>