ax(ueps): align verifyError variable name with idiomatic err convention

Comment on line 126 showed `err` but implementation used `verifyError`,
violating AX Principle 2 (comment must match usage example) and Principle 1
(err is the RFC-sanctioned idiomatic name for local error variables).

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

pkg/ueps/packet_test.go

@@ -138,8 +138,8 @@ func TestPacket_MarshalAndSign_Ugly(t *testing.T) {
 	copy(corrupted, frame)
 	corrupted[len(corrupted)-1] ^= 0xFF
 
-	_, verifyError := ReadAndVerify(bufio.NewReader(bytes.NewReader(corrupted)), sharedSecret)
-	if verifyError == nil {
+	_, err = ReadAndVerify(bufio.NewReader(bytes.NewReader(corrupted)), sharedSecret)
+	if err == nil {
 		t.Error("expected HMAC integrity violation for corrupted frame, got nil")
 	}
 }