Detect IPv6 availability at startup using UDP probe. When IPv6 is not
available: override prefer_ipv6 to false, skip IPv6 DC pings, pass empty
ME v6 proxy map, skip IPv6 coverage checks in health monitor, and skip
IPv6 fallback in upstream health checks. This eliminates useless
connection attempts with timeouts on IPv6-less hosts.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>