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/utils/html-export.test.ts

@@ -58,3 +58,42 @@ describe('buildDataAttrs escapes animation/animationDelay (C2)', () => {
     expect(html).toContain('data-animation-delay="0.5s"');
   });
 });
+
+/**
+ * Adversarial re-review of C1: the `typeName === 'Container' || typeName ===
+ * 'div'` fallback in renderNode (hit when a node's `resolvedName` isn't in
+ * the component resolver, e.g. legacy/tampered saved state) interpolated
+ * `props.tag` raw into `<${tag}` with no validation -- the same breakout
+ * class as the fixed Container.toHtml bug, reachable via deserialized saved
+ * state.
+ */
+describe('renderNode div-fallback allowlists props.tag', () => {
+  const makeUnresolvedDivState = (tag: unknown) =>
+    JSON.stringify({
+      ROOT: {
+        type: { resolvedName: 'div' }, // not in componentResolver -> hits the fallback branch
+        isCanvas: true,
+        props: { tag, style: {} },
+        displayName: 'div',
+        custom: {},
+        hidden: false,
+        nodes: [],
+        linkedNodes: {},
+      },
+    });
+
+  test('a malicious tag value falls back to div -- no injected <img>', () => {
+    const state = makeUnresolvedDivState('div><img src=x onerror=alert(1)');
+    const { html } = exportBodyHtml(state);
+    expect(html).not.toContain('<img');
+    expect(html).not.toContain('onerror');
+    expect(html.startsWith('<div')).toBe(true);
+  });
+
+  test('a valid tag (section) still emits <section', () => {
+    const state = makeUnresolvedDivState('section');
+    const { html } = exportBodyHtml(state);
+    expect(html).toContain('<section');
+    expect(html).toContain('</section>');
+  });
+});