ByteEncoder (the public output encoder) was reached only through the
serializer's UTF-8 default, leaving its transformation-format, single-
byte, BOM, and naming paths untested. Adds direct assertions of the
spec-defined bytes: UTF-8/16/32 (incl. an astral surrogate pair),
ISO-8859-1 with a decimal character-reference fallback for an
unrepresentable scalar, byte-order marks, supports, and canonical names.
