The difficulty minimum for scrypt is absurd. All hashes will satisfy it. It only takes a millisecond.
mplb, _ := hex.DecodeString("fffff00000000000000000000000000000000000000000000000000000000000")
The difficulty should be at least 1 bit of zero on the MSB, which means average 2 hashes to a solution. Even 32 bits isn't nearly low enough. Logically, it should be at least a minute on a budget laptop.