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unpublished; later years not in our dataset yet); y-axis 'Value'→'Score';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/components/compare/CompareAcademics.tsx

@@ -148,6 +148,15 @@ export function CompareAcademics({
       }
       return null;
     });
+    // DfE stopped publishing Progress 8 from 2024/25: those GCSE year groups
+    // sat no KS2 tests (COVID), so there is no baseline to measure progress
+    // from. A bare "No data" reads as a gap on our side — say why.
+    const latestYear = urns.reduce((max, urn) => {
+      const rows = data[String(urn)]?.yearly_data ?? [];
+      const y = rows.length ? Math.trunc(rows[rows.length - 1].year) : 0;
+      return Math.max(max, y);
+    }, 0);
+    const p8NotPublished = latestYear >= 202425;
     const grade5 = latestValues(data, urns, 'english_maths_strong_pass_pct');
     const ebacc = latestValues(data, urns, 'ebacc_entry_pct');
     const att8Anchor = nationalAverages?.secondary?.attainment_8_score;
@@ -189,6 +198,11 @@ export function CompareAcademics({
                 >
                   {banding[i]}
                 </Chip>
+              ) : p8NotPublished ? (
+                <span className={s.small}>
+                  Not published — this GCSE year group sat no KS2 tests (COVID), so DfE has no
+                  baseline to measure progress from
+                </span>
               ) : (
                 <span className={s.small}>No data</span>
               )}
@@ -218,6 +232,18 @@ export function CompareAcademics({
   const national = nationalAverages?.primary;
   const disadvantaged = latestValues(data, urns, 'rwm_expected_disadvantaged_pct');
   const disadvantagedAnchor = benchmarks?.primary?.disadvantaged_rwm_expected_pct ?? null;
+  // Cohort size behind the disadvantaged figure (spec §8.5): these are small
+  // groups where single pupils move the percentage — show roughly how many
+  // pupils the figure rests on.
+  const eligible = latestValues(data, urns, 'eligible_pupils');
+  const disadvantagedShare = latestValues(data, urns, 'disadvantaged_pct');
+  const cohorts = urns.map((_, i) => {
+    const n = eligible[i];
+    const share = disadvantagedShare[i];
+    if (n == null || share == null) return null;
+    const cohort = Math.round((n * share) / 100);
+    return cohort > 0 ? cohort : null;
+  });
 
   return (
     <Section
@@ -270,6 +296,9 @@ export function CompareAcademics({
                     <span className={s.big} style={{ fontSize: '1.1rem' }}>
                       {Math.round(value)}%
                     </span>{' '}
+                    {cohorts[i] != null && (
+                      <span className={s.small}>of ~{cohorts[i]} disadvantaged pupils</span>
+                    )}{' '}
                     {disadvantagedAnchor != null && (
                       <Chip tone={verdict(value, disadvantagedAnchor, 5) === 'below' ? 'warn' : 'good'}>
                         {verdict(value, disadvantagedAnchor, 5) === 'above' &&