feat(destinations): the post-16 section, replacing the placeholder

The 'Post-16 destination data coming soon' note is deleted rather than
reworded: for a school with no sixth form the truthful statement is that
the question does not apply, and a placeholder there implies something is
missing. hasSixthForm and .sixthFormNote go with it — nothing else used them.

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

nextjs-app/components/school/Post16DestinationsSection.tsx

@@ -0,0 +1,40 @@
+/**
+ * Post16DestinationsSection — where a school's Year 13 leavers went.
+ * Server component.
+ *
+ * A separate publication, a separate cohort and a separate question from
+ * After Year 11, so it is a separate section rather than a tab: a parent
+ * choosing a secondary and a student choosing a sixth form are not the same
+ * reader.
+ *
+ * Not rendered at all for a school without post-16 provision. A "no data"
+ * placeholder there would imply something is missing, when the truthful
+ * statement is that the question does not apply — which is why the old
+ * "Post-16 destination data coming soon" note is gone rather than reworded.
+ */
+
+import type { DestinationPhase } from '@/lib/types';
+import { Section, sectionStyles } from './sectionShared';
+import { DestinationsView } from './DestinationsView';
+
+export function Post16DestinationsSection({
+  destinations,
+}: { destinations: DestinationPhase }) {
+  const hasContent = Object.values(destinations.groups)
+    .some(group => (group?.categories?.length ?? 0) > 0);
+  if (!hasContent) return null;
+
+  const cohort = destinations.groups.all?.cohort ?? null;
+  const year = destinations.cohort_year;
+
+  return (
+    <Section id="post16-destinations">
+      <h2 className={sectionStyles.sectionTitle}>After the sixth form</h2>
+      <p className={sectionStyles.sectionSubtitle}>
+        Where {cohort ? `the ${cohort} students` : 'the students'} who finished
+        Year 13{year ? ` in ${year}` : ''} went next.
+      </p>
+      <DestinationsView destinations={destinations} phase="ks5" />
+    </Section>
+  );
+}