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/common/global_ctx.rs

@@ -77,6 +77,11 @@ pub enum GlobalCtxEvent {
 
     ProxyCidrsUpdated(Vec<cidr::Ipv4Cidr>, Vec<cidr::Ipv4Cidr>), // (added, removed)
 
+    UdpBroadcastRelayStartResult {
+        capture_backend: Option<String>,
+        error: Option<String>,
+    },
+
     CredentialChanged,
 }