feat(compare): fill the sticky bar's label rail with a comparison caption

'COMPARING / 3 primary schools' — 0.72rem uppercase eyebrow over a
0.95rem semibold count, sized to sit alongside the 0.92rem chip names
without dominating. Desktop only; mobile pills unchanged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0146VHeLAWjDVE2B5uU67jCB

nextjs-app/__tests__/components/ComparisonView.phase.test.tsx

@@ -72,5 +72,7 @@ test('an all-secondary comparison renders the sections, not an empty primary tab
   });
   expect(screen.getAllByText('Gamma High').length).toBeGreaterThan(0);
   expect(screen.queryByText(/No primary schools in your comparison/)).toBeNull();
+  // The sticky bar's rail caption reflects the active phase and count.
+  expect(screen.getByText('2 secondary schools')).toBeInTheDocument();
   expect(fetchComparison).not.toHaveBeenCalled();
 });