test: add e2e suite for core comparison flows

Adds 17 new specs across six files plus the supporting POM
infrastructure:

- fixtures with auto-opened comparison + typography helpers
- TypographyMenu POM reading values from ComboControl aria-labels
- ComparisonPage extended with side selection, font picking,
  slider-value reader, FontFaceSet inspection, storage snapshot
- compare-flow: A/B selection, aria-pressed state, storage write
- preview-text: input binding + slider character rendering
- slider: keyboard ARIA contract (Arrow / Shift+Arrow / Page / Home / End)
- font-loading: FontFaceSet contains selected, excludes unrelated
- persistence: font selection + typography settings survive reload
- typography: symmetric increase/decrease for all four controls

e2e/compare-flow.test.ts

@@ -0,0 +1,39 @@
+import {
+    expect,
+    test,
+} from './fixtures';
+
+test.describe('compare flow', () => {
+    test('selects fontA and fontB onto opposite sides', async ({ comparison }) => {
+        await comparison.pickPair('Inter', 'Roboto');
+
+        // Each side's header region exposes the font name independently.
+        await expect(comparison.primaryFont).toContainText('Inter');
+        await expect(comparison.secondaryFont).toContainText('Roboto');
+
+        // Slider is rendered and interactive once both fonts are picked.
+        await expect(comparison.slider).toBeVisible();
+    });
+
+    test('reflects active side via aria-pressed', async ({ comparison }) => {
+        await comparison.selectSide('B');
+        expect(await comparison.activeSide()).toBe('B');
+        await expect(comparison.secondarySideButton).toHaveAttribute('aria-pressed', 'true');
+        await expect(comparison.primarySideButton).toHaveAttribute('aria-pressed', 'false');
+    });
+
+    test('persists selection through the comparisonStore localStorage', async ({ comparison }) => {
+        await comparison.pickPair('Inter', 'Roboto');
+
+        // Wait for the store debounce to flush to localStorage.
+        await expect.poll(async () => {
+            const storage = await comparison.readStorage();
+            return storage['glyphdiff:comparison'];
+        }).toMatch(/inter/i);
+
+        const storage = await comparison.readStorage();
+        const state = JSON.parse(storage['glyphdiff:comparison']!);
+        expect(state.fontAId).toBe('inter');
+        expect(state.fontBId).toBe('roboto');
+    });
+});