ax(ueps): rename remaining to payloadBytes for semantic clarity

AX Principle 1: names should convey semantic meaning, not implementation
perspective. `remaining` describes a buffer operation; `payloadBytes`
describes what the data IS — the packet payload content.

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

pkg/ueps/reader.go

@@ -44,11 +44,11 @@ func ReadAndVerify(reader *bufio.Reader, sharedSecret []byte) (*ParsedPacket, er
 		if tagByte == TagPayload {
 			// Payload is length-prefixless; caller frames the stream.
 			// HMAC covers signedData (header TLVs) + raw payload bytes, not the 0xFF tag.
-			remaining, err := io.ReadAll(reader)
+			payloadBytes, err := io.ReadAll(reader)
 			if err != nil {
 				return nil, err
 			}
-			payload = remaining
+			payload = payloadBytes
 			break
 		}