Implement optional JPEG2000 codec adapter (#40)

crates/libremetaverse-openjpeg/README.md

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+# MetaCrate OpenJPEG adapter
+
+This private crate is the unsafe implementation boundary for MetaCrate's
+optional `jpeg2000` feature. Its public surface is safe Rust: native handles,
+callbacks, and OpenJPEG image structures never cross into the imaging API.
+The adapter uses checked arithmetic and bounded memory streams, validates
+header dimensions and component counts before decoded sample-plane allocation,
+and copies decoded samples into owned Rust vectors.
+
+## Native prerequisite
+
+OpenJPEG **2.5.4 or newer** is required. Versions 2.5.1 through 2.5.3 are not
+accepted because they are affected by CVE-2025-54874. No bindgen or libclang
+installation is needed; the crate contains the minimal public ABI declarations
+used by the adapter.
+
+- Linux and other Unix targets: install OpenJPEG development headers/libraries
+  and `pkg-config`; `pkg-config --modversion libopenjp2` must report 2.5.4 or
+  newer.
+- macOS: install `openjpeg` and `pkg-config` (for example with Homebrew). This
+  is the same system-library path used on Linux; no Apple-only API is used.
+- Windows MSVC: install `openjpeg` with vcpkg and expose `VCPKG_ROOT` to Cargo.
+  Use the triplet matching the Rust target and deploy the resulting OpenJPEG
+  DLL beside the application or on its normal DLL search path when using a
+  dynamic triplet.
+
+Enable the adapter with `--features jpeg2000` on either
+`libremetaverse-imaging` or the umbrella `libremetaverse` crate. Without that
+feature, neither the build script nor native library discovery runs.
+
+## License and redistribution
+
+OpenJPEG is BSD-2-Clause licensed. MetaCrate discovers an installation supplied
+by the builder and does not redistribute OpenJPEG source or binaries. A product
+that redistributes an OpenJPEG shared or static library must also satisfy the
+OpenJPEG license's copyright and notice requirements. The ABI declarations in
+this crate follow OpenJPEG's public `openjpeg.h`; MetaCrate's adapter code is
+licensed under the repository's BSD-3-Clause license.