TL;DR. golang.org/x/net/idna.Lookup.ToASCII runs UTS-46 NFKC mapping
on hostnames, which folds 100 non-ASCII Unicode digit codepoints (math
superscripts, circled digits, fullwidth digits, math-styled digits, and
others) to ASCII 0-9. A pre-IDNA net.ParseIP check rejects the
non-ASCII input as not-an-IP, hands it to the library, and gets back...