Implemented hearthbit client side + tests

Common/Client/go/udpsprotocol/protocol.go

@@ -99,6 +99,20 @@ func BuildDisconnectPacket() []byte {
 	})
 }
 
+// BuildAckPacket returns a 17-byte ACK datagram. Unicast clients send it
+// periodically as a keepalive: UDPSServer refreshes the client's last-seen
+// without re-sending CONFIG (which a repeated CONNECT would trigger).
+func BuildAckPacket() []byte {
+	return buildHeader(PacketHeader{
+		Magic:          MagicUDPS,
+		Type:           PktACK,
+		Counter:        0,
+		FragmentIdx:    0,
+		TotalFragments: 1,
+		PayloadBytes:   0,
+	})
+}
+
 // ─── Signal descriptor (136 bytes) ───────────────────────────────────────────
 
 // SignalInfo holds the parsed metadata for one signal.