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/__tests__/lib/utils.test.ts

@@ -269,6 +269,44 @@ describe('isProposedToClose', () => {
   });
 });
 
+describe('isSpecialSchool', () => {
+  const { isSpecialSchool } = require('@/lib/utils');
+
+  it('matches every DfE special-school establishment type', () => {
+    for (const t of [
+      'Community special school',
+      'Foundation special school',
+      'Academy special converter',
+      'Academy special sponsor led',
+      'Non-maintained special school',
+      'Free schools special',
+      'Other independent special school',
+    ]) {
+      expect(isSpecialSchool({ school_type: t })).toBe(true);
+    }
+  });
+
+  it('matches pupil referral units and alternative provision', () => {
+    expect(isSpecialSchool({ school_type: 'Pupil referral unit' })).toBe(true);
+    expect(isSpecialSchool({ school_type: 'Academy alternative provision converter' })).toBe(true);
+  });
+
+  it('does not match mainstream types (no false positives)', () => {
+    for (const t of [
+      'Academy converter',
+      'Community school',
+      'Voluntary aided school',
+      'Free schools',
+      'University technical college',
+      'Other independent school',
+      null,
+      undefined,
+    ]) {
+      expect(isSpecialSchool({ school_type: t })).toBe(false);
+    }
+  });
+});
+
 describe('shortName', () => {
   it('drops the trailing establishment-type words', () => {
     expect(shortName('Barclay Primary School')).toBe('Barclay');