feat(compare): readable comparison chart on mobile

The compare chart squashed clustered schools into a thin band (y pinned
0-100) under an in-chart title + per-school legend that ate ~40% of a
300px card, leaving converging lines indistinguishable on phones.

- Auto-fit the y-axis to the data on all viewports (computeYBounds in
  lib/utils: padded + min-span for percentages, symmetric around 0 for
  progress, fitted for scores; negative pct-named trend metrics are not
  zero-clamped).
- Distinct point style per school (circle/triangle/rect/rectRot/star)
  as secondary encoding for convergence and colour-blindness.
- Mobile: drop in-chart title/legend/axis titles; add a chip row (colour
  dot + name) that doubles as tap-to-focus — highlights one school's
  line and dims the rest. Chart card 300px -> 340px, nearly all plot.
- Fix a latent colour mismatch: datasets were built from Object.entries
  whose integer-like URN keys enumerate in ascending numeric order,
  desyncing line colours from card colours; the chart now receives the
  ordered school list.
- Union years across schools instead of taking the first school's.
- Extract PerformanceChart's matchMedia pattern into hooks/useIsMobile.

Unit tests for metricKind/computeYBounds; e2e journey covers the mobile
chips and focus toggle.

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

e2e/tests/journeys.spec.ts

@@ -91,6 +91,32 @@ test('comparing two schools shows both side by side', async ({ page }) => {
   await expect(page.locator(`a[href*="${urns[1]}"]`).first()).toBeVisible();
 });
 
+test('compare chart on mobile shows school chips with tap-to-focus', async ({ page }) => {
+  await page.setViewportSize({ width: 390, height: 844 });
+
+  await searchByName(page, 'primary');
+  await expect(schoolLinks(page).first()).toBeVisible({ timeout: 15_000 });
+  const hrefs = await schoolLinks(page).evaluateAll((links) =>
+    links.map((l) => (l as HTMLAnchorElement).getAttribute('href') || '')
+  );
+  const urns = [...new Set(hrefs.map((h) => h.match(/\/school\/(\d+)/)?.[1]).filter(Boolean))];
+  expect(urns.length).toBeGreaterThanOrEqual(2);
+
+  await page.goto(`/compare?urns=${urns[0]},${urns[1]}`);
+  await expect(page.locator('canvas:visible').first()).toBeVisible({ timeout: 15_000 });
+
+  // The mobile chart legend renders one chip per school inside the chart card.
+  const chipGroup = page.getByRole('group', { name: /highlight a school/i });
+  const chips = chipGroup.getByRole('button');
+  await expect(chips).toHaveCount(2);
+
+  // Tapping a chip focuses that school's line; tapping again releases it.
+  await chips.first().click();
+  await expect(chips.first()).toHaveAttribute('aria-pressed', 'true');
+  await chips.first().click();
+  await expect(chips.first()).toHaveAttribute('aria-pressed', 'false');
+});
+
 test('rankings page loads a populated table', async ({ page }) => {
   await page.goto('/rankings');
   await expect(page.getByRole('heading', { name: /rankings/i }).first()).toBeVisible();