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unpublished; later years not in our dataset yet); y-axis 'Value'→'Score';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/lib/compareChartData.ts

@@ -53,6 +53,10 @@ export interface CompareChart {
   /** True when England published a 2021/22 figure but no school has one —
    * the UI shows: "DfE didn't publish school-level figures for 2021/22". */
   showUnpublished202122Note: boolean;
+  /** Years where the England overlay has a value but no school does — the
+   * chart shows a dashed-line-only stretch that needs explaining (KS2 and
+   * KS4 have different honest explanations, so the component owns the copy). */
+  englandOnlyYears: number[];
 }
 
 export function buildCompareChart(
@@ -92,11 +96,12 @@ export function buildCompareChart(
     }
   }
 
-  const idx202122 = years.indexOf(202122);
-  const showUnpublished202122Note =
-    idx202122 >= 0 &&
-    englandDataset?.data[idx202122] != null &&
-    schoolDatasets.every((ds) => ds.data[idx202122] == null);
+  const englandOnlyYears = years.filter(
+    (year, i) =>
+      englandDataset?.data[i] != null && schoolDatasets.every((ds) => ds.data[i] == null),
+  );
 
-  return { years, schoolDatasets, englandDataset, showUnpublished202122Note };
+  const showUnpublished202122Note = englandOnlyYears.includes(202122);
+
+  return { years, schoolDatasets, englandDataset, showUnpublished202122Note, englandOnlyYears };
 }