fix(v2): align React Head JSX behavior

[harlan-zw]

🔗 Linked issue

Backports #892, #893, and #894.

❓ Type of change

• 📖 Documentation
• 🐞 Bug fix
• 👌 Enhancement
• ✨ New feature
• 🧹 Chore
• ⚠️ Breaking change

📚 Description

Unhead v2 drops nested React Fragment tags, serializes JSX aliases incorrectly, and cannot map dangerouslySetInnerHTML into head content. Backport the three focused React adapter fixes while retaining v2 lifecycle, async SSR, and escaping behavior.

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[harlan-zw]

🤖 Harlan Agent Kit automated review

This was generated and posted by a robot, not Harlan's personal review.

PASS · 95/100 confidence

• Reviewed 96d18ec9 against v2@9ad444e9
• Base current; GitHub reports no conflicts
• PR title and body match the project template; review threads are resolved
• Adversarial review found no remaining material code issue
• Complete diff reviewed; required CI passed 2/2; confidence includes limited local execution

Human merge decision still required.