feat(rankings): default primary metric to higher standard, not expected

The expected-standard ranking groups too many schools at or near
100% — the leaderboard isn't useful when the top 30+ schools all
share the same value. Switch the default primary metric to the
higher (above-expected) standard, which discriminates between
schools much more clearly at the top end. Secondary still defaults
to Attainment 8. Users can still pick either via the dropdown.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

nextjs-app/components/RankingsView.tsx

@@ -74,7 +74,7 @@ export function RankingsView({
   };
 
   const handlePhaseChange = (phase: string) => {
-    const defaultMetric = phase === 'secondary' ? 'attainment_8_score' : 'rwm_expected_pct';
+    const defaultMetric = phase === 'secondary' ? 'attainment_8_score' : 'rwm_high_pct';
     updateFilters({ phase, metric: defaultMetric });
   };