fix(ui-primitives): prototype-safe alias lookup; pre-warm shiki off the render path

Responding to ds-review-bot round 2 on #662:

- LANG_ALIASES is a Map: an assistant-authored fence label like
  constructor or __proto__ now misses (plain render) instead of resolving
  an inherited object property and crashing shiki mid-conversation. Test
  sweeps the inherited-key labels.
- The singleton is pre-warmed in a deferred task at plugin boot (the
  ~120-175ms engine+grammar construction long task moves off the first
  finalized fence's render); the lazy path remains the correctness
  fallback, and unref keeps non-browser imports from pinning the loop.

Agent Note updated (both languages).

packages/client/ui-primitives/tests/markdown.spec.tsx

@@ -64,6 +64,15 @@ describe('MarkdownText', () => {
     expect(screen.getByRole('link', { name: 'https://deepseek.com' })).toBeTruthy()
   })
 
+  it('a fence labeled with an inherited object key renders plain, never crashing shiki', () => {
+    for (const label of ['constructor', '__proto__', 'toString', 'hasOwnProperty']) {
+      const { container, unmount } = render(<MarkdownText text={'```' + label + '\ncode body\n```'} />)
+      expect(container.querySelector('pre.shiki')).toBeNull()
+      expect(container.querySelector('pre code')?.textContent).toContain('code body')
+      unmount()
+    }
+  })
+
   it('an empty fence keeps the stock pre; a language-less fence renders the plain CodeBlock arm', () => {
     const empty = render(<MarkdownText text={'```\n```'} />)
     expect(empty.container.querySelector('pre')?.outerHTML).toBe('<pre><code></code></pre>')