fix(compare): expert should-fixes S1-S4, S6 — banded chips, P8 reason, KS4 gap caption, all-through framing, cohort sizes

S1: first-choice chip banded (More than half / About 1 in 3 / Over 1 in 4
missed out) so a 44%-offered grammar isn't understated by half.
S2: Progress 8 explains its absence for 2024/25+ cohorts (no KS2 baseline,
COVID) instead of a bare 'No data'.
S3: KS4 trend charts get their own honest gap caption (2019/20-2020/21
unpublished; later years not in our dataset yet); y-axis 'Value'→'Score';
buildCompareChart exposes englandOnlyYears.
S4: all-through schools labelled in chips, rail caption says 'N schools ·
<phase> view' for mixed baskets, whole-school roll no longer judged
against the single-phase median, community section carries an all-ages
caveat.
S6 (spec §8.5): disadvantaged attainment shows the cohort behind it
('of ~50 disadvantaged pupils').

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

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

@@ -73,6 +73,15 @@ describe('buildCompareChart', () => {
     expect(eng.data[chart.years.indexOf(202122)]).toBe(58.7);
   });
 
+  it('lists England-only years so the component can caption dashed-only stretches', () => {
+    const chart = buildCompareChart(THREE_SCHOOLS, SCHOOL_LIST, 'rwm_expected_pct', {
+      202122: 58.7,
+    });
+    expect(chart.englandOnlyYears).toEqual([202122]);
+    const none = buildCompareChart(THREE_SCHOOLS, SCHOOL_LIST, 'rwm_expected_pct');
+    expect(none.englandOnlyYears).toEqual([]);
+  });
+
   it('flags the unpublished 2021/22 school-level year when England has data but schools do not', () => {
     const withNational = buildCompareChart(THREE_SCHOOLS, SCHOOL_LIST, 'rwm_expected_pct', {
       202122: 58.7,