ax(ueps): expand MarshalAndSign comment to full usage example with error handling

AX Principle 2: comments show concrete usage with realistic values, not
minimal call-site only. Single-line comment omitted error handling and
downstream use — now shows the complete call pattern.

Co-Authored-By: Charon <charon@lethean.io>

pkg/ueps/packet.go

@@ -61,6 +61,8 @@ func NewPacketBuilder(intentID uint8, payload []byte) *PacketBuilder {
 }
 
 // frame, err := builder.MarshalAndSign([]byte("my-shared-secret"))
+// if err != nil { return nil, err }
+// conn.Write(frame) // send signed frame over network
 func (builder *PacketBuilder) MarshalAndSign(sharedSecret []byte) ([]byte, error) {
 	frameBuffer := new(bytes.Buffer)