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unpublished; later years not in our dataset yet); y-axis 'Value'→'Score';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

@@ -48,10 +48,25 @@ export function CompareCommunity({
     );
   };
 
+  const anyAllThrough = schools.some((school) => /all.?through/i.test(school.phase ?? ''));
+
   return (
     <Section
       title="Who goes there"
-      how="The school's community, from the latest school census. State-school averages are computed from our dataset and shown for context — there's no “right” number here."
+      how={
+        <>
+          The school&apos;s community, from the latest school census. State-school averages are
+          computed from our dataset and shown for context — there&apos;s no &ldquo;right&rdquo;
+          number here.
+          {anyAllThrough && (
+            <>
+              {' '}
+              For all-through schools these figures cover the whole school, all ages — not just
+              the {isSecondary ? 'secondary' : 'primary'} phase.
+            </>
+          )}
+        </>
+      }
     >
       <SectionGrid schools={schools}>
         <Measure label="Pupils on roll">