Discovery failing on IPv4 only setups

[michaelherger]

This code stems from #1366, which said "expose to both IPv4 and IPv6":

librespot/discovery/src/server.rs

Lines 284 to 289 in 9c7d756

	let address = if cfg!(windows) {
	SocketAddr::new(Ipv4Addr::UNSPECIFIED.into(), *port)
	} else {
	// this creates a dual stack socket on non-windows systems
	SocketAddr::new(Ipv6Addr::UNSPECIFIED.into(), *port)
	};

Unfortunately it breaks discovery on systems set up with IPv4 only (eg. piCorePlayer.org, a TinyCore based distribution):

[2026-08-12T19:15:56Z WARN  libmdns] Failed to register IPv6 receiver: Os { code: 97, kind: Uncategorized, message: "Address family not supported by protocol" }

If I hardcode to using IPv4 only, it would run.

Shouldn't the code gracefully fall back to IPv4 in case IPv6 was not available?