fix: Reject NaN when serializing a decimal

[youdie006]

marshalDecimal rejects values outside the 12-digit range, but every comparison with NaN is false, so NaN passes the check (decimal.go:27) and FormatFloat writes NaN:

s, err := httpsfv.Marshal(httpsfv.NewItem(math.NaN())) // "NaN", <nil>
it, err := httpsfv.UnmarshalItem([]string{"NaN"})      // parses as a Token, <nil>

So a NaN float is sent as a Token without an error. RFC 9651 section 4.1.5 step 1 says to fail if the input is not a decimal number; +Inf and -Inf already return ErrInvalidDecimal. This adds math.IsNaN(d) to the same check, and a NaN row to TestMarshalDecimal that fails on main. NaN gets the same error text as the infinities.

go test -race, go vet, gofmt and GOARCH=386 go test pass on Go 1.26; I did not run oldstable.

Written with AI assistance (Claude); I have reviewed the change.