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
