fix: implement RFC 5155 closest-encloser + next-closer NSEC3 chain validation

[poyrazK]

Summary

• Rewrite ValidateNSEC3Proof with full RFC 5155 Section 7.2.1 NXDOMAIN proof chain:
  • Find closest-encloser by walking query labels longest→shortest (findClosestEncloserNSEC3)
  • Find next-closer NSEC3 (smallest hash > closest-encloser) and verify hash range covers query
  • Verify no wildcard exists at (closest-encloser + 1 label) (wildcard denial)
  • No-data responses handled via exact-name NSEC3 type bitmap check (early exit)
• Add new error types: ErrNSEC3NoClosestEncloser, ErrNSEC3NoNextCloser
• Add helper functions: findClosestEncloserNSEC3, nextCloserNSEC3, wildcardName, nsec3HashLessThan

Fixes: #102

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Reviewing files that changed from the base of the PR and between b1b16cc and 3385f0e.

📒 Files selected for processing (1)

• internal/dns/packet/dnssec_verify.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/nsec3-nxdomain-validation

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[poyrazK]

It's okay to merge