Port the plug-in to Rust

Replace the C++ plug-in and the arma-file-formats-cxx bridge with a Rust
plug-in using the gimp-rs bindings (../gimp-rs) and arma-file-formats
directly.

- Prefer the PAA loader over GIMP 3.2's own file-paa, which cannot read
  DXT1 (priority -1, no magics).
- Export the merged visible image via the export options, converting gray
  and indexed images, instead of only the top layer.
- Load RGB textures, validate decoded data sizes.
- Disable export metadata, PAA cannot store it and GIMP logged an error.
- Keep the existing file when an export fails.
- Install with gimptool (GIMP 3.2 no longer reads ~/.config/GIMP/3.0).
- Add headless GIMP integration tests.
- License: GPL-2.0-or-later.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: TheWillard

.gitignore

@@ -1,62 +1 @@
-# C++
-
-# Prerequisites
-*.d
-
-# Compiled Object files
-*.slo
-*.lo
-*.o
-*.obj
-
-# Precompiled Headers
-*.gch
-*.pch
-
-# Linker files
-*.ilk
-
-# Debugger Files
-*.pdb
-
-# Compiled Dynamic libraries
-*.so
-*.dylib
-*.dll
-
-# Fortran module files
-*.mod
-*.smod
-
-# Compiled Static libraries
-*.lai
-*.la
-*.a
-*.lib
-
-# Executables
-*.exe
-*.out
-*.app
-
-# debug information files
-*.dwo
-
-# CMake
-
-CMakeLists.txt.user
-CMakeCache.txt
-CMakeFiles
-CMakeScripts
-Testing
-Makefile
-cmake_install.cmake
-install_manifest.txt
-compile_commands.json
-CTestTestfile.cmake
-_deps
-CMakeUserPresets.json
-
-# Custom
-/build
-/.vscode
+/target