Functionally equivalent to [`crate::codecs::zstd::Zstd`] when the plane's state bytes are empty: the encoder emits a plain Zstd frame and the decoder accepts it. When `state_bytes` is non-empty, the codec treats it as a pre-trained Zstd dictionary. Encoding seeds the compressor's LZ window with the dictionary content so repeated structural patterns (e.g. attention block headers, embedding row layouts) compress to shorter back-references. Decoding must use the *same* dictionary bytes, which is why they ride along in the plane record's state slot. Dictionary training is out of scope for this module. See `zstd --train` or the `ptwm chains train-dict` future CLI surface. The codec is generic over whatever bytes the caller decides to ship as the dictionary.