v0.5.0: Windows/ARM64 and Linux/ARM64 cross targets, .deb/.msi packaging

- Cross toolchains: llvm-mingw-aarch64 (Windows ARM64; SDL3 staged from MSYS2's
  clangarm64 repo since SDL ships no MinGW ARM64 dev package) and
  clang-aarch64-linux-gnu (Linux ARM64; arm64 libc++/SDL3 via dpkg multiarch).
- Distro images: Dockerfile.linux-arm64, Dockerfile.win-arm64, Dockerfile.debian
  and Dockerfile.debian-arm64 (Debian 13 with upstream CMake, whose 3.31 gate
  cannot enable import std).
- Packaging via CPack: .deb for Ubuntu 26.04 and Debian 13 on amd64 and arm64
  (dependencies derived by dpkg-shlibdeps), portable .zip/.tar.gz, and a
  Windows .msi built with wixl (msitools) straight from the Linux cross image.
  wixl 0.106 lacks arm64, so Windows/ARM64 ships the portable zip only.
- CMake: parameterize the SDL3 MinGW triple, add install() + CPack config.
- CI: build/package jobs for both arm64 targets and both Debian packages;
  existing Linux/Windows jobs now also emit the .deb / .zip + .msi.
- scripts/: build-linux-arm64, build-windows-arm64, build-debian,
  build-debian-arm64.

Dockerfile.win

@@ -31,6 +31,7 @@ RUN --mount=type=cache,target=/var/lib/apt,sharing=locked \
         curl \
         ninja-build \
         python3 \
+        wixl \
         xz-utils \
     && rm -rf /var/lib/apt/lists/*
 
@@ -54,7 +55,9 @@ COPY . .
 RUN cmake -S . -B build/windows -G Ninja \
         -DCMAKE_TOOLCHAIN_FILE=cmake/toolchains/llvm-mingw-x86_64.cmake \
         -DCMAKE_BUILD_TYPE=Release \
-    && cmake --build build/windows -j
+    && cmake --build build/windows -j \
+    && cmake --build build/windows --target openra3_msi \
+    && (cd build/windows && cpack -G ZIP)
 
 # package target: the .exe plus the SDL3 runtime DLL.
 FROM ubuntu:26.04 AS package