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

backend/tests/test_national_averages_marts.py

@@ -25,6 +25,11 @@ class _Ks2Row:
     year = LATEST
     rwm_expected_pct = 62.1
     gps_expected_pct = 72.0
+    # Per-subject higher-standard nationals — reading/maths reach the higher
+    # standard, writing is teacher-assessed at greater depth (writing_gd_pct).
+    reading_high_pct = 29.0
+    writing_gd_pct = 13.0
+    maths_high_pct = 24.0
 
 
 class _Ks4Row:
@@ -84,6 +89,16 @@ def test_ks4_averages_come_from_the_mart_not_the_dataframe(payload):
     assert body["by_year"][-1]["secondary"]["progress_8_score"] == -0.02
 
 
+def test_per_subject_higher_standard_nationals_are_surfaced(payload):
+    # The SATs chart compares each bar to its own benchmark, so the per-subject
+    # higher-standard / greater-depth nationals must reach the payload — not
+    # only the combined rwm_high_pct.
+    body = payload(_StubSession)
+    assert body["primary"]["reading_high_pct"] == 29.0
+    assert body["primary"]["writing_gd_pct"] == 13.0
+    assert body["primary"]["maths_high_pct"] == 24.0
+
+
 def test_ks4_secondary_empty_when_mart_missing(payload):
     # No computed stand-in: the UI labels national figures as official DfE
     # data, so an empty mart must yield an empty secondary series.