fix(builder): sanitize non-string style values + allowlist element tags (XSS)

Adversarial re-review found the C1 fix incomplete plus an adjacent
same-class XSS, both reachable via the AI update_props path and
deserialized saved state:

- cssPropsToString only ran sanitizeCssValue on typeof-string values, so a
  non-string style value (array/object) with a valid key skipped
  sanitization entirely and was template-coerced raw into style="...",
  e.g. { color: ['red', '"><img src=x onerror=alert(1)>'] }. Now every
  non-number value is coerced with String() and sanitized; numbers stay
  raw. sanitizeBreakoutChars also now escapes < and > (previously only ;
  and ") as defense-in-depth, since values can reach it from non-string
  sources.

- props.tag (Container) and props.level (Heading) were interpolated raw
  into the tag position of exported HTML (`<${tag}`, `<${level}`) with no
  runtime validation, letting a malicious value break out of the tag
  entirely. Both are now allowlisted/clamped against their known-safe sets
  (div/section/article/header/footer/main; h1-h6), falling back to
  div/h2. Applied in Container's live render + toHtml, Heading's live
  render + toHtml, and the typeName==='div' fallback branch in
  html-export.ts's renderNode (hit for unresolved/legacy node types).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/components/layout/Container.toHtml.test.ts

@@ -33,3 +33,33 @@ describe('Container.toHtml cssId/cssClass', () => {
     expect(html).toContain('id="anchor-id"');
   });
 });
+
+describe('Container.toHtml tag allowlist (adversarial re-review, same class as C1)', () => {
+  test('a malicious tag value falls back to div -- no injected <img>, no broken-out attrs', () => {
+    const { html } = toHtml({ tag: 'div><img src=x onerror=alert(1)' }, 'child');
+    expect(html).not.toContain('<img');
+    expect(html).not.toContain('onerror');
+    expect(html.startsWith('<div')).toBe(true);
+    expect(html.endsWith('</div>')).toBe(true);
+  });
+
+  test('a tag value outside the known-safe set falls back to div', () => {
+    const { html } = toHtml({ tag: 'script' }, 'child');
+    expect(html.startsWith('<div')).toBe(true);
+    expect(html).not.toContain('<script');
+  });
+
+  test('a valid tag (section) still emits <section', () => {
+    const { html } = toHtml({ tag: 'section' }, 'child');
+    expect(html).toContain('<section');
+    expect(html).toContain('</section>');
+  });
+
+  test('all other allowlisted tags still work', () => {
+    for (const tag of ['div', 'article', 'header', 'footer', 'main']) {
+      const { html } = toHtml({ tag }, 'child');
+      expect(html.startsWith(`<${tag}`)).toBe(true);
+      expect(html.endsWith(`</${tag}>`)).toBe(true);
+    }
+  });
+});