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/app.py

@@ -807,6 +807,10 @@ async def get_la_averages(request: Request):
 _KS2_NATIONAL_METRICS = [
     "rwm_expected_pct", "rwm_high_pct",
     "reading_expected_pct", "writing_expected_pct", "maths_expected_pct",
+    # Per-subject higher-standard nationals: reading/maths reach the "higher
+    # standard" in the tests; writing is teacher-assessed at "greater depth"
+    # (writing_gd_pct). Needed so each SATs bar compares to its own benchmark.
+    "reading_high_pct", "writing_gd_pct", "maths_high_pct",
     "gps_expected_pct", "gps_high_pct", "science_expected_pct",
     "reading_avg_score", "maths_avg_score", "gps_avg_score",
     "reading_progress", "writing_progress", "maths_progress",