fix(compare): auto-phase tab now also syncs metric to match detected phase

The useEffect that auto-selects the Primary/Secondary tab was calling
setComparePhase() without updating selectedMetric. When a URL carries a
primary metric (e.g. metric=rwm_expected_pct) but the shortlisted schools
are secondary, the tab would switch to Secondary while the metric stayed
at rwm_expected_pct — which is null for all secondary schools, causing
every card to show "–" and requiring a manual tab toggle to fix.

Fix: after determining the phase from school data, check whether the
current metric belongs to that phase's category list. If not, reset to
the phase default (attainment_8_score for secondary, rwm_expected_pct
for primary). A metric that already fits the phase (e.g. the URL already
carried attainment_8_score) is preserved unchanged.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nextjs-app/components/ComparisonView.tsx

@@ -118,14 +118,22 @@ export function ComparisonView({
   const primarySchools = selectedSchools.filter(s => classifySchool(s) === 'primary');
   const secondarySchools = selectedSchools.filter(s => classifySchool(s) === 'secondary');
 
-  // Auto-select tab with more schools
+  // Auto-select tab with more schools and sync the metric to match the detected phase.
+  // This fixes the case where the URL carries a primary metric (e.g. rwm_expected_pct)
+  // but the shortlisted schools are secondary — the phase tab switches but the metric
+  // needs to follow, otherwise all secondary cards show "–" for a primary-only field.
   useEffect(() => {
-    if (comparisonData && selectedSchools.length > 0) {
-      if (secondarySchools.length > primarySchools.length) {
-        setComparePhase('secondary');
-      } else {
-        setComparePhase('primary');
-      }
+    if (!comparisonData || selectedSchools.length === 0) return;
+    const newPhase = secondarySchools.length > primarySchools.length ? 'secondary' : 'primary';
+    setComparePhase(newPhase);
+    // Only reset the metric when it doesn't belong to the newly detected phase.
+    // This preserves a correct metric that came from the URL (e.g. metric=attainment_8_score).
+    const phaseCategories = newPhase === 'secondary' ? SECONDARY_CATEGORIES : PRIMARY_CATEGORIES;
+    const metricFitsPhase = metrics.some(
+      (m) => m.key === selectedMetric && phaseCategories.includes(m.category)
+    );
+    if (!metricFitsPhase) {
+      setSelectedMetric(newPhase === 'secondary' ? 'attainment_8_score' : 'rwm_expected_pct');
     }
   }, [comparisonData]); // eslint-disable-line react-hooks/exhaustive-deps