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.module.css

@@ -44,30 +44,46 @@
   background: var(--bg-secondary, #f3ede4);
 }
 
-/* ── National average marker ── */
-.natLine {
+/* ── Per-bar national average marker ──
+   Each bar compares against its own benchmark (expected vs higher standard /
+   greater depth), so the marker sits on the individual bar's track rather than
+   as one line spanning both bars. */
+.natTick {
   position: absolute;
-  top: 0;
-  height: calc(100% - 20px);
-  width: 1.5px;
-  background: rgba(224, 114, 86, 0.35); /* --accent-coral at 35% */
-  z-index: 2;
+  top: -3px;
+  bottom: -3px;
+  width: 2px;
+  transform: translateX(-50%);
+  background: var(--accent-coral, #e07256);
+  border-radius: 2px;
+  z-index: 4;
   pointer-events: none;
 }
 
-.natPill {
+.natTick::before {
+  content: '';
   position: absolute;
-  top: -10px;
+  top: -3px;
+  left: 50%;
   transform: translateX(-50%);
+  width: 5px;
+  height: 5px;
+  border-radius: 50%;
   background: var(--accent-coral, #e07256);
-  color: #fff;
+}
+
+.barHeaderRight {
+  display: flex;
+  align-items: baseline;
+  gap: 0.4rem;
+}
+
+.natLabel {
   font-size: 0.55rem;
   font-weight: 700;
-  padding: 0.1rem 0.35rem;
-  border-radius: 4px;
-  white-space: nowrap;
-  z-index: 3;
-  letter-spacing: 0.02em;
+  color: var(--accent-coral-dark, #b04a2e);
+  text-transform: uppercase;
+  letter-spacing: 0.03em;
 }
 
 /* ── Bar rows ── */