See `entropy::huff_llm` for the bit-field decomposition. The codec gates on `Role::Raw` terminals (see `accepts` for why role, not `element_width`), interprets the plane as little-endian `u16` words, trials both the FP16 {1,5,5,5} and BF16 {1,4,4,7} layouts, and keeps the smaller (the chosen layout tag travels in `state_bytes`). Odd-length or incompressible planes fall back to a raw store, so the codec is always total. It is self-delimiting (the word count is framed in the payload) and ignores the container's `decoded_len`.