Services registered via the service API have a domain but no internal
domain (that's a Wild Cloud instance concept). The dnsmasq generator
was producing broken entries like `local=//` and `address=//`.
Fix: skip internal domain entries when InternalDomain is empty, in
both config.go and config_modular.go. Also fix DNS entries to point
to Central's IP (where HAProxy listens) rather than the backend
address — all traffic flows through Central.
Added 3 tests covering: service-only, mixed services+instances.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Paul Payne
e4e1a15f92