refactor(detail): extract derived flags and nav items to lib

Pure data-shape logic moves out of the client components so page.tsx can
compute the section list on the server without importing them.

The secondary page is not a variant of the primary one -- different section
ids (gcse, wellbeing), different flags, and History gated on more than one
year rather than at least one -- so it gets its own computeSecondaryFlags and
buildSecondaryNavItems rather than bending a shared function.

Adds 16 unit tests covering the all-through and special-school branches,
previously reachable only through a full component render.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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

@@ -0,0 +1,144 @@
+import {
+  computeSchoolFlags, buildNavItems,
+  computeSecondaryFlags, buildSecondaryNavItems,
+} from '@/lib/schoolSections';
+import {
+  primaryFixture, secondaryFixture, allThroughFixture, specialFixture,
+} from '../support/schoolFixtures';
+
+describe('computeSchoolFlags', () => {
+  it('treats an all-through school as both primary-capable and secondary', () => {
+    const f = computeSchoolFlags(allThroughFixture);
+    expect(f.isAllThrough).toBe(true);
+    expect(f.isSecondary).toBe(true);
+    expect(f.hasKS2Results).toBe(true);
+    expect(f.hasKS4Results).toBe(true);
+  });
+
+  it('classifies an ordinary primary', () => {
+    const f = computeSchoolFlags(primaryFixture);
+    expect(f.isPrimary).toBe(true);
+    expect(f.isSecondary).toBe(false);
+    expect(f.hasKS2Results).toBe(true);
+    expect(f.hasKS4Results).toBe(false);
+  });
+
+  it('classifies an ordinary secondary', () => {
+    const f = computeSchoolFlags(secondaryFixture);
+    expect(f.isSecondary).toBe(true);
+    expect(f.isAllThrough).toBe(false);
+    expect(f.hasKS4Results).toBe(true);
+  });
+
+  it('suppresses the KS2 comparison for a special school', () => {
+    const f = computeSchoolFlags(specialFixture);
+    expect(f.isSpecial).toBe(true);
+    expect(f.suppressKs2Comparison).toBe(true);
+    expect(f.suppressKs4Comparison).toBe(true);
+  });
+
+  it('treats an all-zero KS2 row as a placeholder', () => {
+    const f = computeSchoolFlags(specialFixture);
+    expect(f.ks2Placeholder).toBe(true);
+  });
+
+  it('does not suppress comparison for an ordinary primary', () => {
+    const f = computeSchoolFlags(primaryFixture);
+    expect(f.isSpecial).toBe(false);
+    expect(f.suppressKs2Comparison).toBe(false);
+  });
+
+  it('detects a gender split only when census counts are present', () => {
+    expect(computeSchoolFlags(primaryFixture).hasGenderSplit).toBe(true);
+    expect(
+      computeSchoolFlags({ ...primaryFixture, census: null }).hasGenderSplit,
+    ).toBe(false);
+  });
+});
+
+describe('buildNavItems', () => {
+  it('omits sections with no data', () => {
+    const flags = computeSchoolFlags(specialFixture);
+    const ids = buildNavItems(flags, {
+      ofsted: null, admissions: null, yearlyDataLength: 1,
+    }).map((n) => n.id);
+
+    expect(ids).not.toContain('ofsted');
+    expect(ids).not.toContain('admissions');
+    expect(ids).toContain('history');
+  });
+
+  it('labels the results section by phase', () => {
+    const label = (fixture: any) => {
+      const flags = computeSchoolFlags(fixture);
+      return buildNavItems(flags, {
+        ofsted: fixture.ofsted,
+        admissions: fixture.admissions,
+        yearlyDataLength: fixture.yearlyData.length,
+      }).find((n) => n.id === 'results')?.label;
+    };
+
+    expect(label(primaryFixture)).toBe('SATs');
+    expect(label(secondaryFixture)).toBe('GCSEs');
+    expect(label(allThroughFixture)).toBe('Results');
+  });
+
+  it('keeps the engagement-led ordering', () => {
+    const flags = computeSchoolFlags(primaryFixture);
+    const ids = buildNavItems(flags, {
+      ofsted: primaryFixture.ofsted,
+      admissions: primaryFixture.admissions,
+      yearlyDataLength: primaryFixture.yearlyData.length,
+    }).map((n) => n.id);
+
+    expect(ids).toEqual([
+      'ofsted', 'results', 'admissions', 'inclusion',
+      'history', 'school-life', 'local-area', 'finances',
+    ]);
+  });
+});
+
+describe('computeSecondaryFlags', () => {
+  it('reads results and sixth form from the secondary fixture', () => {
+    const f = computeSecondaryFlags(secondaryFixture);
+    expect(f.hasResults).toBe(true);
+    expect(f.hasSixthForm).toBe(false);
+    expect(f.hasWellbeing).toBe(true);
+  });
+
+  it('reports sixth form when GIAS flags it', () => {
+    expect(computeSecondaryFlags(allThroughFixture).hasSixthForm).toBe(true);
+  });
+
+  it('suppresses the comparison for a special school', () => {
+    const f = computeSecondaryFlags(specialFixture);
+    expect(f.isSpecial).toBe(true);
+    expect(f.suppressComparison).toBe(true);
+  });
+
+  it('does not flag Progress 8 as suspended for pre-2024/25 cohorts', () => {
+    expect(computeSecondaryFlags(secondaryFixture).p8Suspended).toBe(false);
+  });
+});
+
+describe('buildSecondaryNavItems', () => {
+  it('uses the secondary section ids', () => {
+    const flags = computeSecondaryFlags(secondaryFixture);
+    const ids = buildSecondaryNavItems(flags, {
+      ofsted: secondaryFixture.ofsted,
+      admissions: secondaryFixture.admissions,
+      yearlyDataLength: secondaryFixture.yearlyData.length,
+    }).map((n) => n.id);
+
+    expect(ids).toEqual(['ofsted', 'gcse', 'admissions', 'history', 'wellbeing', 'finances']);
+  });
+
+  it('gates History on more than one year, unlike the primary page', () => {
+    const flags = computeSecondaryFlags(secondaryFixture);
+    const ids = buildSecondaryNavItems(flags, {
+      ofsted: null, admissions: null, yearlyDataLength: 1,
+    }).map((n) => n.id);
+
+    expect(ids).not.toContain('history');
+  });
+});