fix(net): fallback multicast join without adapters

[Skyline-23]

This pull request improves network interface detection and multicast socket setup for sandboxed platforms where ifaddr may return no adapters. When adapters are present, behavior is unchanged. The fallback uses a dummy UDP connect (no traffic sent) solely to learn the default local IPv4.

Network interface detection improvements

• get_private_addresses now probes the default IPv4 via a dummy UDP connect when no adapters are reported, keeping loopback filtering rules.

Multicast socket setup enhancements

• mcast_socket falls back to the probed default IPv4 for IP_MULTICAST_IF/IP_ADD_MEMBERSHIP, with a final 0.0.0.0 membership attempt if probing fails.
• Binding remains "", avoiding port conflicts while only multicast membership targets the detected address.