Extend ByteEncoder to the multi-byte CJK encodings (Shift-JIS, EUC-JP,
EUC-KR, GBK, GB18030, Big5). Each inverse is built by running the existing,
tested whole-buffer decoder over every valid byte sequence and recording
the shortest sequence per scalar, so the encoder is by construction the
exact inverse of the decoder, with no new data or separate encode tables.
GB18030's astral range uses the four-byte pointer formula, so every scalar
encodes. Round-trip tests cover Japanese, Korean, Chinese, and a GB18030
astral character.
Closes#106
Add PureXML.Parsing.ByteEncoder, the inverse of ByteDecoder, and a
Serializer.serialize(_:encoding:) / PureXML.serialize(_:encoding:) that
emits bytes in a chosen encoding (the libxml2 save-with-encoding model).
Covers the Unicode transformation formats and the single-byte legacy
families, built by inverting the existing vendored to-Unicode tables, so
no new data is introduced. The declaration carries the canonical encoding
name, a UTF-16/32 stream is preceded by a byte-order mark, and any
unrepresentable scalar becomes a decimal numeric character reference, so
output round-trips. CJK output encodings remain (tracked on #106).
Part of #106, #105
