j2k-types

Shared JPEG 2000 encode-stage contract types for the j2k workspace.

This crate is the neutral public contract between the j2k adapter surface and the j2k-native codec engine: encode-stage job, output, and report types are defined once here so neither crate mirrors the other's definitions. It contains plain data types only — codec behavior lives in j2k-native, and the encode-stage accelerator traits stay in their owning crates.