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

e2e/tests/journeys.spec.ts

@@ -204,6 +204,8 @@ test('comparing two schools shows the parent-first sections side by side', async
     .locator('[aria-label="Schools in this comparison"]')
     .evaluate((el) => getComputedStyle(el).gridTemplateColumns);
   expect(barTemplate).toMatch(/^200px /);
+  // ...and its label rail carries the comparison caption.
+  await expect(page.getByText(/^\d+ (primary|secondary) schools?$/)).toBeVisible();
 
   // Ofsted linkout goes to the school's provider page, never a report deep-link
   const ofstedLink = page.getByRole('link', { name: /Ofsted page/i }).first();