This module gathers everything PTWM does at the *information-theoretic* layer of compression: - [`shannon`] / [`histogram`] — measure per-plane entropy in bits/byte. The PPG layer uses these to decide whether a chain is worth running; the explorer uses them to score candidate plane factorisations. - [`huffman`] and [`rans`] — two pure-Rust byte-oriented entropy coders with the same public interface (`compress` / `decompress` / `compress_outcome`) and a peer position in the dispatcher's per-plane trial encode. Both share the 4-stream interleaved framing in [`stream_frame`] and return the [`outcome::CompressOutcome`] enum so the plane-codec wrapper can distinguish "encoded N bytes" from "not beneficial" / "incompressible" / "dst too small" / "len overflow". Neither codec is the default. The dispatcher trial-encodes every plane against both (plus Identity and Zstd) and keeps the smallest payload, so the per-plane winner is determined by the data shape rather than a configuration choice.