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.module.css

@@ -895,6 +895,71 @@
   }
 }
 
+/* ── RWM bridge ("Why is combined lower?") ── */
+.rwmBridge {
+  display: flex;
+  align-items: flex-start;
+  gap: 0.75rem;
+  margin: 0.5rem 0 1.5rem;
+  padding: 0.9rem 1rem;
+  background: var(--bg-secondary, #f3ede4);
+  border-radius: 8px;
+}
+
+.rwmBridgeIcon {
+  flex-shrink: 0;
+  width: 22px;
+  height: 22px;
+  border-radius: 50%;
+  background: var(--accent-coral, #e07256);
+  color: #fff;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  font-weight: 700;
+  font-size: 0.8rem;
+  font-family: var(--font-playfair), 'Playfair Display', Georgia, serif;
+  margin-top: 0.05rem;
+}
+
+.rwmBridgeBody {
+  flex: 1;
+  min-width: 0;
+}
+
+.rwmBridgeText {
+  font-size: 0.85rem;
+  color: var(--text-secondary, #5c564d);
+  line-height: 1.5;
+}
+
+.rwmBridgeText strong {
+  color: var(--text-primary, #1a1612);
+  font-weight: 700;
+}
+
+.rwmBridgeMath {
+  display: flex;
+  gap: 0.35rem;
+  margin-top: 0.35rem;
+  flex-wrap: wrap;
+  font-family: var(--font-playfair), 'Playfair Display', Georgia, serif;
+  color: var(--text-muted, #6d685f);
+  font-size: 0.78rem;
+  font-style: italic;
+  align-items: baseline;
+}
+
+.rwmBridgeMath strong {
+  color: var(--text-primary, #1a1612);
+  font-weight: 700;
+  font-style: normal;
+}
+
+.rwmBridgeMathSep {
+  opacity: 0.5;
+}
+
 /* ── Progress scores row (below SatsChart) ── */
 .progressScoresRow {
   margin-top: 1.25rem;

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={[
                   {