fix(detail): filter Ofsted sentinel codes, show sixth-form judgement, label carried-forward grades

Batch A of applying compare-screen learnings to the detail page (points 2-3):

- Point 2: the legacy OEIF sub-judgement grid rendered any non-null value
  via OFSTED_LABELS[value], so Ofsted's sentinel code 9 ('not applicable')
  leaked as an undefined label, and the sixth-form judgement was dropped
  entirely. New shared ofstedLegacyAreas() helper returns only real grades
  (1-4) and includes Sixth Form provision. Both detail views + the
  all-same-grade check use it.
- Point 3: the OEIF disclaimer always claimed 'Ofsted no longer makes an
  overall judgement' even when showing a carried-forward grade. It's now
  grade_source-aware and labels carried-forward grades as such, matching
  the compare screen.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0146VHeLAWjDVE2B5uU67jCB

nextjs-app/__tests__/lib/utils.test.ts

@@ -9,6 +9,7 @@ import {
   isValidPostcode,
   debounce,
   buildOfstedListBadge,
+  ofstedLegacyAreas,
   metricKind,
   shortName,
   computeYBounds,
@@ -111,6 +112,35 @@ describe('debounce', () => {
   });
 });
 
+describe('ofstedLegacyAreas', () => {
+  it('returns only real grades (1-4) and drops sentinel/null values', () => {
+    const areas = ofstedLegacyAreas({
+      quality_of_education: 1,
+      behaviour_attitudes: 2,
+      personal_development: null,
+      leadership_management: 9, // sentinel "not applicable" — must be dropped
+      early_years_provision: 9, // secondary school — not applicable
+      sixth_form_provision: 2,
+    });
+    expect(areas.map((a) => a.label)).toEqual([
+      'Quality of Teaching',
+      'Behaviour in School',
+      'Sixth Form',
+    ]);
+    expect(areas.every((a) => a.value >= 1 && a.value <= 4)).toBe(true);
+  });
+
+  it('includes the sixth-form judgement where present (was previously dropped)', () => {
+    const areas = ofstedLegacyAreas({ sixth_form_provision: 1 });
+    expect(areas).toEqual([{ label: 'Sixth Form', value: 1 }]);
+  });
+
+  it('includes Early Years for a primary school', () => {
+    const areas = ofstedLegacyAreas({ quality_of_education: 2, early_years_provision: 1 });
+    expect(areas.map((a) => a.label)).toContain('Early Years (Reception)');
+  });
+});
+
 describe('buildOfstedListBadge', () => {
   it('returns grade word + year for OEIF Outstanding', () => {
     const badge = buildOfstedListBadge({ ofsted_grade: 1, ofsted_date: '2023-11-15', ofsted_framework: 'OEIF' });