fix(unhead): preserve numeric zero head content

[harlan-zw]

🔗 Linked issue

Backport of #885.

❓ Type of change

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

📚 Description

Unhead v2 discarded numeric zero in safe input, title rendering, and inferred SEO metadata. A private hasContent predicate now owns presence checks, while empty, false, null, and non-finite values remain omitted.

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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 009a84c6 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.