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/preview-text.test.ts

@@ -0,0 +1,21 @@
+import {
+    expect,
+    test,
+} from './fixtures';
+
+test.describe('preview text', () => {
+    test('drives the slider character rendering', async ({ comparison }) => {
+        await comparison.pickPair('Inter', 'Roboto');
+        await comparison.setPreviewText('Sphinx');
+
+        // Each grapheme renders as a `.char-wrap` cell in the slider once
+        // both fonts are loaded. Six glyphs → six cells.
+        await expect(comparison.slider.locator('.char-wrap')).toHaveCount(6);
+    });
+
+    test('preserves the typed value in the input', async ({ comparison }) => {
+        const text = 'Sphinx of black quartz';
+        await comparison.setPreviewText(text);
+        await expect(comparison.previewInput).toHaveValue(text);
+    });
+});