Add portable Windows build and update GC runtime

README.md

@@ -37,7 +37,9 @@ workspace/
 └── openroller/
 ```
 
-Install SDL3, OpenGL, libpng and GLM, then build:
+Install CMake, a C++ compiler, and OpenGL development files, then build. SDL3,
+GLM, and the PNG decoder are fetched automatically when no installed package
+is available:
 
 ```sh
 cmake -S openroller -B openroller/build -DCMAKE_BUILD_TYPE=Release
@@ -51,6 +53,33 @@ with `VECTORAIL_CORE_SOURCE_DIR` and `VECTORAIL_GC_SOURCE_DIR`.
 
 The raw stage inspection utility is built as `openroller-stage-probe`.
 
+### Windows
+
+To cross-build a self-contained x86-64 package from Linux with MinGW-w64:
+
+```sh
+cmake -S openroller -B openroller/build-windows \
+  -DCMAKE_TOOLCHAIN_FILE=openroller/cmake/toolchains/mingw-x86_64.cmake \
+  -DCMAKE_BUILD_TYPE=Release
+cmake --build openroller/build-windows --parallel
+cmake --install openroller/build-windows \
+  --prefix openroller/build-windows/package
+```
+
+For a native MSYS2 CLANG64 build, install CMake, Ninja, Clang, SDL3, GLM, and
+OpenGL packages in the CLANG64 shell, then run:
+
+```sh
+cmake -S openroller -B openroller/build-windows -G Ninja \
+  -DCMAKE_BUILD_TYPE=Release
+cmake --build openroller/build-windows --parallel
+cmake --install openroller/build-windows \
+  --prefix openroller/build-windows/package
+```
+
+The package directory contains `OpenRoller.exe`, `SDL3.dll`,
+`openroller.cfg`, and the shaders needed at runtime.
+
 ## PSP build
 
 The PSP port is built with the `pspdev/pspdev` container: