feat(destinations): types and secondary section flags

Destinations are secondary-only, so the flags go on computeSecondaryFlags
rather than computeSchoolFlags. A phase counts as present only when some
pupil group carries categories — an empty block would otherwise open a nav
entry pointing at a section that never renders.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BvdDKvFFSZuMVDH5fEyTob

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

@@ -0,0 +1,84 @@
+import { computeSecondaryFlags, buildSecondaryNavItems } from '@/lib/schoolSections';
+import type { School, SchoolDestinations } from '@/lib/types';
+
+const schoolInfo = {
+  urn: 137083, school_name: 'Northbrook Academy', phase: 'Secondary',
+  has_sixth_form: true,
+} as unknown as School;
+
+const base = { schoolInfo, yearlyData: [], deprivation: null, finance: null };
+
+const phase = (categories = 1) => ({
+  cohort_year: '2022/23',
+  groups: {
+    all: {
+      cohort: 180,
+      categories: Array.from({ length: categories }, () => ({
+        category: 'school_sixth_form' as const,
+        pupils: 75, percentage: 41.7, status: 'published' as const,
+      })),
+      aggregates: {},
+    },
+  },
+});
+
+const ks4Only: SchoolDestinations = { ks4: phase(), ks5: null };
+const both: SchoolDestinations = { ks4: phase(), ks5: phase() };
+
+describe('computeSecondaryFlags — destinations', () => {
+  it('flags KS4 destinations when the block carries categories', () => {
+    const flags = computeSecondaryFlags({ ...base, destinations: ks4Only });
+    expect(flags.hasKs4Destinations).toBe(true);
+    expect(flags.hasKs5Destinations).toBe(false);
+  });
+
+  it('flags both phases when both are present', () => {
+    const flags = computeSecondaryFlags({ ...base, destinations: both });
+    expect(flags.hasKs4Destinations).toBe(true);
+    expect(flags.hasKs5Destinations).toBe(true);
+  });
+
+  it('flags neither when the block is absent', () => {
+    const flags = computeSecondaryFlags({ ...base, destinations: null });
+    expect(flags.hasKs4Destinations).toBe(false);
+    expect(flags.hasKs5Destinations).toBe(false);
+  });
+
+  it('does not flag a phase whose groups carry no categories', () => {
+    const empty: SchoolDestinations = {
+      ks4: { cohort_year: '2022/23', groups: {} }, ks5: null,
+    };
+    expect(computeSecondaryFlags({ ...base, destinations: empty }).hasKs4Destinations)
+      .toBe(false);
+  });
+
+  it('does not flag a phase whose only group has an empty category list', () => {
+    const empty: SchoolDestinations = { ks4: phase(0), ks5: null };
+    expect(computeSecondaryFlags({ ...base, destinations: empty }).hasKs4Destinations)
+      .toBe(false);
+  });
+});
+
+describe('buildSecondaryNavItems — destinations', () => {
+  const navInput = {
+    ofsted: null, admissions: null, admissionDistance: null,
+    hasLocation: false, yearlyDataLength: 0,
+  };
+
+  it('adds both entries, after GCSEs', () => {
+    const flags = computeSecondaryFlags({ ...base, destinations: both });
+    const ids = buildSecondaryNavItems({ ...flags, hasResults: true }, navInput)
+      .map(i => i.id);
+    expect(ids).toContain('destinations');
+    expect(ids).toContain('post16-destinations');
+    expect(ids.indexOf('destinations')).toBeGreaterThan(ids.indexOf('gcse'));
+    expect(ids.indexOf('post16-destinations')).toBe(ids.indexOf('destinations') + 1);
+  });
+
+  it('adds no entry for a phase that will not render — the nav must not link to a missing anchor', () => {
+    const flags = computeSecondaryFlags({ ...base, destinations: null });
+    const ids = buildSecondaryNavItems(flags, navInput).map(i => i.id);
+    expect(ids).not.toContain('destinations');
+    expect(ids).not.toContain('post16-destinations');
+  });
+});