fix(detail): compare each SATs bar to its own national benchmark

The KS2 SATs chart drew a single national-average line spanning the
full height of each subject's chart area, positioned at the national
*expected* value. But the area stacks two bars — Expected and Exceeding
— and the higher-standard/greater-depth national is a very different,
much lower figure (e.g. reading higher standard ~29% vs expected ~75%).
So the line crossed the Exceeding bar at the wrong place, making every
school's exceeding result look far below national when it wasn't.

The per-subject higher-standard nationals were already computed in the
fact_ks2_national_averages mart; they just weren't serialized. Fix:

- backend: add reading_high_pct, writing_gd_pct (writing = greater
  depth) and maths_high_pct to the national-averages payload.
- SchoolDetailView: pass a nationalExceedingPct per subject, mapping
  writing to the greater-depth figure.
- SatsChart: replace the single full-height line with a national marker
  on each bar's own track (coral tick + "nat X%" in the bar header), so
  Expected and Exceeding each sit against the correct benchmark.

KS2 only; the secondary Attainment 8 chart already uses one line for
one measure and is untouched.

Verified: tsc --noEmit, next build, and backend pytest (national
averages marts, incl. a new test guarding the per-subject nationals).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0146VHeLAWjDVE2B5uU67jCB

nextjs-app/components/SatsChart.tsx

@@ -8,6 +8,9 @@ interface SubjectData {
   expectedPct: number | null;
   exceedingPct: number | null;
   nationalExpectedPct: number | null;
+  /** National higher-standard / greater-depth benchmark for the exceeding
+   * bar — a different figure from the expected national. */
+  nationalExceedingPct: number | null;
 }
 
 interface SatsChartProps {
@@ -17,11 +20,45 @@ interface SatsChartProps {
 const RULER_TICKS = [0, 25, 50, 75, 100];
 const GRIDLINE_POSITIONS = [25, 50, 75];
 
+/** One results bar with a national marker positioned on its own track, so the
+ * expected and exceeding bars each compare against their own benchmark. */
+function BarRow({
+  label,
+  value,
+  national,
+  barRef,
+  barClass,
+}: {
+  label: string;
+  value: number;
+  national: number | null;
+  barRef: React.RefObject<HTMLDivElement | null>;
+  barClass: string;
+}) {
+  return (
+    <div className={styles.barRow}>
+      <div className={styles.barHeader}>
+        <span className={styles.barLabelSuffix}>{label}</span>
+        <span className={styles.barHeaderRight}>
+          {national != null && <span className={styles.natLabel}>nat {national.toFixed(0)}%</span>}
+          <span className={styles.barLabel}>{value.toFixed(0)}%</span>
+        </span>
+      </div>
+      <div className={styles.barTrack}>
+        <div ref={barRef} className={`${styles.bar} ${barClass}`} data-width={value} />
+        {national != null && (
+          <div className={styles.natTick} style={{ left: `${national}%` }} aria-hidden="true" />
+        )}
+      </div>
+    </div>
+  );
+}
+
 function SubjectColumn({ subject }: { subject: SubjectData }) {
   const expectedRef = useRef<HTMLDivElement>(null);
   const exceedingRef = useRef<HTMLDivElement>(null);
 
-  const { name, expectedPct, exceedingPct, nationalExpectedPct } = subject;
+  const { name, expectedPct, exceedingPct, nationalExpectedPct, nationalExceedingPct } = subject;
 
   // Animate bars on mount
   useEffect(() => {
@@ -51,44 +88,25 @@ function SubjectColumn({ subject }: { subject: SubjectData }) {
           ))}
         </div>
 
-        {/* National average marker */}
-        {nationalExpectedPct != null && (
-          <div className={styles.natLine} style={{ left: `${nationalExpectedPct}%` }}>
-            <div className={styles.natPill}>{nationalExpectedPct.toFixed(0)}%</div>
-          </div>
-        )}
-
-        {/* Bars */}
+        {/* Bars — each carries its own national marker */}
         <div className={styles.barGroup}>
           {expectedPct != null && (
-            <div className={styles.barRow}>
-              <div className={styles.barHeader}>
-                <span className={styles.barLabelSuffix}>Expected</span>
-                <span className={styles.barLabel}>{expectedPct.toFixed(0)}%</span>
-              </div>
-              <div className={styles.barTrack}>
-                <div
-                  ref={expectedRef}
-                  className={`${styles.bar} ${styles.barExpected}`}
-                  data-width={expectedPct}
-                />
-              </div>
-            </div>
+            <BarRow
+              label="Expected"
+              value={expectedPct}
+              national={nationalExpectedPct}
+              barRef={expectedRef}
+              barClass={styles.barExpected}
+            />
           )}
           {exceedingPct != null && (
-            <div className={styles.barRow}>
-              <div className={styles.barHeader}>
-                <span className={styles.barLabelSuffix}>Exceeding</span>
-                <span className={styles.barLabel}>{exceedingPct.toFixed(0)}%</span>
-              </div>
-              <div className={styles.barTrack}>
-                <div
-                  ref={exceedingRef}
-                  className={`${styles.bar} ${styles.barExceeding}`}
-                  data-width={exceedingPct}
-                />
-              </div>
-            </div>
+            <BarRow
+              label="Exceeding"
+              value={exceedingPct}
+              national={nationalExceedingPct}
+              barRef={exceedingRef}
+              barClass={styles.barExceeding}
+            />
           )}
         </div>
 
@@ -135,7 +153,7 @@ export default function SatsChart({ subjects }: SatsChartProps) {
           Exceeding / high score
         </div>
         <div className={styles.legendItem}>
-          <div className={styles.legendSwatch} style={{ background: 'var(--accent-coral, #e07256)', borderRadius: '50%' }} />
+          <div className={styles.legendSwatch} style={{ background: 'var(--accent-coral, #e07256)', width: '3px', height: '12px', borderRadius: '2px' }} />
           National average
         </div>
       </div>