feat(site-builder): add dependency-free formatHtml prettifier

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

craft/src/utils/format-html.test.ts

@@ -0,0 +1,41 @@
+import { describe, test, expect } from 'vitest';
+import { formatHtml } from './format-html';
+
+describe('formatHtml', () => {
+  test('indents nested block elements two spaces per level', () => {
+    expect(formatHtml('<div><section><p>hi</p></section></div>')).toBe(
+      '<div>\n  <section>\n    <p>hi</p>\n  </section>\n</div>',
+    );
+  });
+
+  test('leaves inline tags on the same line as their text', () => {
+    expect(formatHtml('<p>hello <strong>world</strong> now</p>')).toBe(
+      '<p>hello <strong>world</strong> now</p>',
+    );
+  });
+
+  test('void elements do not open an indent level', () => {
+    expect(formatHtml('<div><img src="a.png"><br><p>x</p></div>')).toBe(
+      '<div>\n  <img src="a.png">\n  <br>\n  <p>x</p>\n</div>',
+    );
+  });
+
+  test('preserves <pre> contents verbatim', () => {
+    const src = '<div><pre>  keep\n   this</pre></div>';
+    expect(formatHtml(src)).toBe('<div>\n  <pre>  keep\n   this</pre>\n</div>');
+  });
+
+  test('is idempotent', () => {
+    const once = formatHtml('<div><section><p>hi</p></section></div>');
+    expect(formatHtml(once)).toBe(once);
+  });
+
+  test('empty and whitespace-only input round-trip to an empty string', () => {
+    expect(formatHtml('')).toBe('');
+    expect(formatHtml('   \n  ')).toBe('');
+  });
+
+  test('unbalanced markup never produces negative indent', () => {
+    expect(formatHtml('</div><p>x</p>')).toBe('</div>\n<p>x</p>');
+  });
+});