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

e2e/tests/journeys.spec.ts

@@ -175,8 +175,30 @@ test('an all-through school shows BOTH its KS2 SATs and its GCSE results, not ju
   // The combined results section carries both phases.
   const results = page.locator('#results');
   await expect(results.getByText(/SATs & GCSE Results/)).toBeVisible();
-  await expect(results.getByText('Reading, Writing & Maths combined')).toBeVisible(); // KS2
-  await expect(results.getByText('Attainment 8').first()).toBeVisible(); // KS4
+  await expect(results.getByRole('heading', { name: /Primary.*KS2 SATs/ })).toBeVisible(); // KS2 block
+  await expect(results.getByRole('heading', { name: /Secondary.*GCSEs/ })).toBeVisible(); // KS4 block
+});
+
+test('a special school is not shown as failing against the mainstream England average', async ({ page }) => {
+  // Special schools sit the same tests but very few pupils reach the mainstream
+  // "expected standard", so a "0.0% · −62 pts below England average" rendering
+  // portrays them as failing against a benchmark that doesn't fit. The results
+  // section drops the England comparison and explains the context instead.
+  const SP_URN = 101099; // Greenmead School — a community special school
+  const res = await page.request.get(`/api/schools/${SP_URN}`);
+  expect(res.ok()).toBeTruthy();
+  const detail = await res.json();
+  test.skip(
+    !/special|pupil referral|alternative provision/i.test(detail.school_info?.school_type ?? ''),
+    'precondition: chosen URN must currently be a special school',
+  );
+
+  await page.goto(`/school/${SP_URN}`);
+  const results = page.locator('#results');
+  // The special-school context note is shown…
+  await expect(results.getByText(/This is a special school/i)).toBeVisible({ timeout: 15_000 });
+  // …and the mainstream England-average comparison is dropped entirely.
+  await expect(results.getByText(/England avg/i)).toHaveCount(0);
 });
 
 test('school with no performance data still gets a working detail page', async ({ page }) => {