fix: don't portray special schools as failing the mainstream benchmark

Special schools, PRUs and alternative provision teach pupils with SEND who
sit the same KS2/KS4 assessments but very few reach the mainstream "expected
standard". Their headline attainment is therefore ~0% (or a very low
Attainment 8), and the site was comparing that to the England average and
painting it red — e.g. Greenmead School (a community special school) rendered
as "0.0% — −62 pts below England average" with three 0% red SATs bars. That
portrays a special school as catastrophically failing against a benchmark
that doesn't fit it.

Add a shared `isSpecialSchool()` helper (detects every DfE special-school
establishment type — all contain "special" — plus PRUs / alternative
provision) and drop the mainstream England comparison + "below" framing for
these schools across every surface:

- Detail (primary + secondary): a plain-English context note explaining the
  school is special and why the comparison isn't shown; England-average delta
  chips, "England avg" hints, the SATs national markers, the Attainment-8
  "vs national" bar and the trend chart's England overlay are all suppressed.
  An all-zero placeholder SATs row hides the (empty) subject bar chart and the
  "why is combined lower" bridge.
- Rankings / search rows (primary + secondary): the mainstream RWM / Attainment
  8 stat shows "—" with no "vs national" delta, instead of "0% · −62 vs
  national".
- Compare: special schools' attainment values are dropped (no misleading dot
  at 0% / no "Below England average" chip); progress banding, which IS a fair
  measure for special schools, is kept.

Belt-and-braces zero-guard: a whole-row zero attainment (special or a
suppressed cohort) is also treated as not-comparable, while a legitimate
single 0 (e.g. 0% exceeding at a mainstream school) stays comparable.

Tests: new isSpecialSchool unit tests (every DfE special type matched, no
mainstream false positives); an e2e journey asserts Greenmead shows the
special-school note and no England-average comparison. tsc clean; 108/108 unit.

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

nextjs-app/components/SecondarySchoolDetailView.module.css

@@ -395,6 +395,22 @@
     line-height: 1.5;
 }
 
+/* Special-school context note: soft, informational (teal), not a warning. */
+.specialNote {
+    background: var(--bg-secondary, #f3ede4);
+    border-left: 4px solid var(--accent-teal, #2d7d7d);
+    border-radius: 0 6px 6px 0;
+    padding: 0.7rem 1rem;
+    margin: 0 0 1rem;
+    font-size: 0.9rem;
+    line-height: 1.5;
+    color: var(--text-secondary, #5c564d);
+    max-width: 72ch;
+}
+.specialNote strong {
+    color: var(--text-primary, #1a1612);
+}
+
 /* ── Metrics Grid & Cards ────────────────────────────── */
 .metricsGrid {
     display: grid;