Parse NTP unicast servers with SourceRef

[WilliamK112]

Summary

• add an NtpServer IR model plus VrpConfig.ntp_servers
• parse both ntp-service unicast-server and older ntp unicast-server forms with SourceRef-backed address and optional vpn-instance
• add sample config coverage and tests/test_ntp.py for syntax variants, vpn-instance, SourceRef, and serialization

Fixes #39

Validation

• PYTHONPATH=src python -m unittest discover -s tests -p test_ntp.py -v ✅ (6 passed)
• PYTHONPATH=src python -m unittest discover -s tests -v ✅ (153 passed)
• uvx ruff check src tests ✅
• git diff --check ✅

[zynovex-support]

Thank you, @WilliamK112 — and congrats on the project's first external contribution! 🎉

This is exactly the kind of PR maintainers hope for. It follows the codebase conventions precisely (top-level source: SourceRef alongside the Traced[...] field, dispatch placement next to the other one-line globals, _col() for provenance, no garbage facts), handles both ntp-service unicast-server and the older ntp unicast-server form, captures the optional vpn-instance, and the tests cover the syntax variants, vpn-instance, SourceRef line/col, and serialization.

Verified locally and in CI: 153 tests pass (3.9 + 3.12), ruff clean. Merging now — thanks for jumping on #39.

If you'd like another in the same vein, #37 (parse snmp-agent community) and #41 (parse info-center loghost) are open and structured the same way. 🙌