feat: add sidebar and breadcrumb CSS exports

Closes #3

tests/breadcrumb.test.ts

@@ -0,0 +1,13 @@
+import { describe, it, expect } from 'vitest';
+import { readFileSync } from 'fs';
+
+const css = readFileSync(new URL('../dist/breadcrumb.css', import.meta.url), 'utf-8');
+
+describe('breadcrumb.css', () => {
+  it('is non-empty', () => { expect(css.length).toBeGreaterThan(0); });
+  it('references --fg', () => { expect(css).toContain('var(--fg)'); });
+  it('references --fg-muted', () => { expect(css).toContain('var(--fg-muted)'); });
+  it('styles aria-current="page"', () => { expect(css).toContain('[aria-current="page"]'); });
+  it('styles .breadcrumb-sep', () => { expect(css).toContain('.breadcrumb-sep'); });
+  it('styles .breadcrumb links', () => { expect(css).toContain('.breadcrumb a'); });
+});