feat(ui): replace card grid with row-based results and plain-language metric labels

Replace the school card grid with a scannable row list that shows 3x more
results per screen. Each row shows: school name + R,W&M % with trend,
area/type meta, and reading/writing/maths progress scores with plain-English
band labels (e.g. "above average") instead of raw numbers.

Add lib/metrics.ts as a single source of truth for plain-language metric
explanations and the progressBand() helper. Map view toggle is unchanged.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nextjs-app/lib/metrics.ts

@@ -0,0 +1,113 @@
+/**
+ * Plain-language metric labels and explanations.
+ * Single source of truth for all metric display strings.
+ */
+
+export interface MetricExplanation {
+  label: string;
+  plain: string;
+  detail?: string;
+}
+
+export const METRIC_EXPLANATIONS: Record<string, MetricExplanation> = {
+  rwm_expected_pct: {
+    label: 'Reading, Writing & Maths',
+    plain: '% of pupils achieving the expected standard in all three subjects at age 11',
+    detail: 'The national average is around 60%. Higher means more pupils reached the expected level.',
+  },
+  rwm_high_pct: {
+    label: 'Higher Standard — Reading, Writing & Maths',
+    plain: '% of pupils exceeding the expected level in all three subjects',
+    detail: 'A more demanding threshold. The national average is around 8%.',
+  },
+  reading_expected_pct: {
+    label: 'Reading — Expected Standard',
+    plain: '% of pupils achieving the expected standard in reading at age 11',
+  },
+  writing_expected_pct: {
+    label: 'Writing — Expected Standard',
+    plain: '% of pupils achieving the expected standard in writing at age 11',
+  },
+  maths_expected_pct: {
+    label: 'Maths — Expected Standard',
+    plain: '% of pupils achieving the expected standard in maths at age 11',
+  },
+  reading_high_pct: {
+    label: 'Reading — Higher Standard',
+    plain: '% of pupils exceeding the expected level in reading',
+  },
+  writing_high_pct: {
+    label: 'Writing — Higher Standard',
+    plain: '% of pupils exceeding the expected level in writing',
+  },
+  maths_high_pct: {
+    label: 'Maths — Higher Standard',
+    plain: '% of pupils exceeding the expected level in maths',
+  },
+  gps_expected_pct: {
+    label: 'Grammar & Spelling',
+    plain: '% of pupils achieving the expected standard in grammar, punctuation & spelling',
+  },
+  science_expected_pct: {
+    label: 'Science',
+    plain: '% of pupils achieving the expected standard in science',
+  },
+  reading_progress: {
+    label: 'Reading Progress',
+    plain: 'How much pupils improved in reading compared to similar schools',
+    detail: '0 = national average. Positive means better-than-average progress from Year 2 to Year 6.',
+  },
+  writing_progress: {
+    label: 'Writing Progress',
+    plain: 'How much pupils improved in writing compared to similar schools',
+    detail: '0 = national average. Positive means better-than-average progress from Year 2 to Year 6.',
+  },
+  maths_progress: {
+    label: 'Maths Progress',
+    plain: 'How much pupils improved in maths compared to similar schools',
+    detail: '0 = national average. Positive means better-than-average progress from Year 2 to Year 6.',
+  },
+  reading_avg_score: {
+    label: 'Reading Average Score',
+    plain: 'Average scaled score in the reading test (range 80–120, average = 100)',
+  },
+  maths_avg_score: {
+    label: 'Maths Average Score',
+    plain: 'Average scaled score in the maths test (range 80–120, average = 100)',
+  },
+  gps_avg_score: {
+    label: 'Grammar & Spelling Average Score',
+    plain: 'Average scaled score in the grammar, punctuation & spelling test',
+  },
+  overall_absence_pct: {
+    label: 'Absence Rate',
+    plain: '% of school sessions missed due to any reason',
+    detail: 'Lower is better. The national average is around 5%.',
+  },
+  persistent_absence_pct: {
+    label: 'Persistent Absence',
+    plain: '% of pupils missing 10% or more of school sessions',
+    detail: 'Lower is better. Persistent absence can significantly affect attainment.',
+  },
+  rwm_expected_disadvantaged_pct: {
+    label: 'Disadvantaged Pupils — Reading, Writing & Maths',
+    plain: '% of disadvantaged pupils achieving the expected standard in all three subjects',
+  },
+  disadvantaged_gap: {
+    label: 'Disadvantaged Gap',
+    plain: 'Difference in attainment between disadvantaged pupils and their peers',
+    detail: 'A smaller gap means the school is doing more to support disadvantaged pupils.',
+  },
+};
+
+/**
+ * Returns a plain-English band label for a KS2 progress score.
+ * Progress scores are centred at 0 (= national average).
+ */
+export function progressBand(score: number): string {
+  if (score > 3) return 'well above average';
+  if (score > 1) return 'above average';
+  if (score >= -1) return 'around average';
+  if (score >= -3) return 'below average';
+  return 'well below average';
+}