Preserve generated zone TTL directive

[Mirochill]

Closes #44

Summary

• add an explicit $TTL 3600 directive when the script creates a new zone file
• restore $TTL 3600 after zone.to_file(...) rewrites the temporary zone file without a TTL directive
• keep the existing SOA/NS TTL value aligned with the default TTL used for generated RPZ files

Validation

• Not run locally
• git diff --check HEAD~1..HEAD
• git show --stat --oneline --summary HEAD
• Static inspection of update-zonefile.py