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/SecondarySchoolRow.tsx

@@ -11,7 +11,7 @@
 'use client';
 
 import type { School } from '@/lib/types';
-import { buildOfstedListBadge, getPhaseStyle, schoolUrl, formatAgeRange, isProposedToClose } from '@/lib/utils';
+import { buildOfstedListBadge, getPhaseStyle, schoolUrl, formatAgeRange, isProposedToClose, isSpecialSchool } from '@/lib/utils';
 import styles from './SecondarySchoolRow.module.css';
 
 function detectAdmissionsTag(school: School): string | null {
@@ -55,8 +55,12 @@ export function SecondarySchoolRow({
   const ofstedBadge = buildOfstedListBadge(school);
   const phase = getPhaseStyle(school.phase);
   const att8 = school.attainment_8_score;
+  // Special schools / PRUs / AP: Attainment 8 vs the LA average isn't a fair
+  // comparison (their pupils have SEND), so drop the delta and the number
+  // rather than show them trailing "the average" by design.
+  const att8Comparable = att8 != null && att8 !== 0 && !isSpecialSchool(school);
   const laDelta =
-    att8 != null && laAvgAttainment8 != null ? att8 - laAvgAttainment8 : null;
+    att8Comparable && laAvgAttainment8 != null ? (att8 as number) - laAvgAttainment8 : null;
 
   const admissionsTag = detectAdmissionsTag(school);
   const sixthForm = hasSixthForm(school);
@@ -106,7 +110,7 @@ export function SecondarySchoolRow({
         <div className={styles.line3}>
           <span className={styles.stat}>
             <strong className={styles.statValueLarge}>
-              {att8 != null ? att8.toFixed(1) : '—'}
+              {att8Comparable ? (att8 as number).toFixed(1) : '—'}
             </strong>
             <span className={styles.statLabel}>Attainment 8</span>
           </span>