初版功能完成

internal/subnet/header.go

@@ -0,0 +1,9 @@
+// Package subnet transports complete raw IPv4 packets inside Overlay UDP.
+package subnet
+
+import "remlink/internal/protocol"
+
+// EncodeDatagram uses the one authoritative v1 SessionHeader codec.
+func EncodeDatagram(sessionID uint64, packet []byte) ([]byte, error) {
+	return protocol.EncodeIPv4Session(sessionID, packet)
+}