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/core.rs

@@ -484,6 +484,15 @@ struct NetworkOptions {
     )]
     disable_upnp: Option<bool>,
 
+    #[arg(
+        long,
+        env = "ET_ENABLE_UDP_BROADCAST_RELAY",
+        help = t!("core_clap.enable_udp_broadcast_relay").to_string(),
+        num_args = 0..=1,
+        default_missing_value = "true"
+    )]
+    enable_udp_broadcast_relay: Option<bool>,
+
     #[arg(
         long,
         env = "ET_RELAY_ALL_PEER_RPC",
@@ -1142,6 +1151,9 @@ impl NetworkOptions {
             .disable_sym_hole_punching
             .unwrap_or(f.disable_sym_hole_punching);
         f.disable_upnp = self.disable_upnp.unwrap_or(f.disable_upnp);
+        f.enable_udp_broadcast_relay = self
+            .enable_udp_broadcast_relay
+            .unwrap_or(f.enable_udp_broadcast_relay);
         // Configure tld_dns_zone: use provided value if set
         if let Some(tld_dns_zone) = &self.tld_dns_zone {
             f.tld_dns_zone = tld_dns_zone.clone();