feat: add Windows UDP broadcast relay (#2222)

This may helps games to find rooms in virtual network.

- add opt-in Windows UDP broadcast relay config flag and CLI/env plumbing
- capture local UDP broadcasts with Windows raw sockets, normalize packets, and inject them via PeerManager

easytier/src/instance/mod.rs

@@ -10,3 +10,6 @@ pub mod proxy_cidrs_monitor;
 
 #[cfg(feature = "tun")]
 pub mod virtual_nic;
+
+#[cfg(any(windows, test))]
+pub(crate) mod windows_udp_broadcast;