feat(sats): add "why is combined lower?" bridge between hero and cascade

Explains the intersection semantics of RWM combined — a pupil is only
counted if they met the bar in all three subjects — with a math line
showing the per-subject percentages collapsing to the combined figure.
Only renders when all four values are present; national-average pill
markers on the cascade are untouched.

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

nextjs-app/components/SchoolDetailView.tsx

@@ -562,6 +562,30 @@ export function SchoolDetailView({
                 )}
               </div>
 
+              {latestResults.rwm_expected_pct != null &&
+                latestResults.reading_expected_pct != null &&
+                latestResults.writing_expected_pct != null &&
+                latestResults.maths_expected_pct != null && (
+                  <div className={styles.rwmBridge}>
+                    <span className={styles.rwmBridgeIcon} aria-hidden="true">?</span>
+                    <div className={styles.rwmBridgeBody}>
+                      <div className={styles.rwmBridgeText}>
+                        Why is combined lower? A pupil is only counted if they met the bar in{' '}
+                        <strong>all three</strong> subjects. Some passed reading but not writing; some passed writing but not maths.
+                      </div>
+                      <div className={styles.rwmBridgeMath}>
+                        <span>Reading <strong>{latestResults.reading_expected_pct.toFixed(0)}%</strong></span>
+                        <span className={styles.rwmBridgeMathSep}>·</span>
+                        <span>Writing <strong>{latestResults.writing_expected_pct.toFixed(0)}%</strong></span>
+                        <span className={styles.rwmBridgeMathSep}>·</span>
+                        <span>Maths <strong>{latestResults.maths_expected_pct.toFixed(0)}%</strong></span>
+                        <span className={styles.rwmBridgeMathSep}>→</span>
+                        <span>All three <strong>{latestResults.rwm_expected_pct.toFixed(0)}%</strong></span>
+                      </div>
+                    </div>
+                  </div>
+                )}
+
               <SatsChart
                 subjects={[
                   {