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.