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/SchoolDetailView.tsx

@@ -752,18 +752,22 @@ export function SchoolDetailView({
                       expectedPct: latestResults.reading_expected_pct,
                       exceedingPct: latestResults.reading_high_pct,
                       nationalExpectedPct: suppressKs2Comparison ? null : primaryAvg.reading_expected_pct,
+                      nationalExceedingPct: suppressKs2Comparison ? null : primaryAvg.reading_high_pct,
                     },
                     {
                       name: 'Writing',
                       expectedPct: latestResults.writing_expected_pct,
                       exceedingPct: latestResults.writing_high_pct,
                       nationalExpectedPct: suppressKs2Comparison ? null : primaryAvg.writing_expected_pct,
+                      // Writing's higher level is teacher-assessed "greater depth".
+                      nationalExceedingPct: suppressKs2Comparison ? null : primaryAvg.writing_gd_pct,
                     },
                     {
                       name: 'Maths',
                       expectedPct: latestResults.maths_expected_pct,
                       exceedingPct: latestResults.maths_high_pct,
                       nationalExpectedPct: suppressKs2Comparison ? null : primaryAvg.maths_expected_pct,
+                      nationalExceedingPct: suppressKs2Comparison ? null : primaryAvg.maths_high_pct,
                     },
                   ]}
                 />