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.linux-arm64

@@ -0,0 +1,84 @@
+# syntax=docker/dockerfile:1
+
+# Linux on ARM64 cross-build environment (aarch64-linux-gnu), isolated from the
+# x86_64 Linux image and the Windows images.
+#
+# Toolchain: clang-21 + libc++ with the arm64 libc++ (for `import std;`), the
+# arm64 glibc from `gcc-aarch64-linux-gnu`, arm64 SDL3 from `libsdl3-dev:arm64`,
+# and lld as the linker. The arm64 packages are installed with dpkg multiarch
+# (`arm64` added to the sources); the native clang/cmake build tools stay amd64.
+
+FROM ubuntu:26.04 AS dev
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+ARG APT_MIRROR=""
+RUN if [ -n "$APT_MIRROR" ]; then \
+        sed -i -e "s|http://archive.ubuntu.com/ubuntu|${APT_MIRROR}|g" \
+               -e "s|http://security.ubuntu.com/ubuntu|${APT_MIRROR}|g" \
+               -e "s|http://ports.ubuntu.com/ubuntu-ports|${APT_MIRROR}|g" \
+               /etc/apt/sources.list /etc/apt/sources.list.d/*.sources 2>/dev/null || true; \
+    fi
+
+# Enable the arm64 architecture on top of the emulated amd64 userland.
+RUN dpkg --add-architecture arm64
+
+RUN --mount=type=cache,target=/var/lib/apt,sharing=locked \
+    --mount=type=cache,target=/var/cache/apt,sharing=locked \
+    for attempt in 1 2 3 4 5; do \
+        apt-get -o Acquire::Retries=5 update && break; \
+        echo "apt-get update failed (attempt $attempt), retrying"; \
+        sleep 5; \
+    done \
+    && apt-get -o Acquire::Retries=5 install -y --no-install-recommends \
+        ca-certificates \
+        clang-21 \
+        cmake \
+        dpkg-dev \
+        file \
+        gcc-aarch64-linux-gnu \
+        libc++-21-dev:arm64 \
+        libc++abi-21-dev:arm64 \
+        libsdl3-dev:arm64 \
+        libunwind-21-dev:arm64 \
+        lld-21 \
+        ninja-build \
+        pkg-config \
+        python3 \
+    && rm -rf /var/lib/apt/lists/*
+
+ENV CC=clang-21
+ENV CXX=clang++-21
+# Point pkg-config at the arm64 packages so SDL3 resolves to the aarch64 build.
+ENV PKG_CONFIG_LIBDIR=/usr/lib/aarch64-linux-gnu/pkgconfig
+
+WORKDIR /work
+COPY . .
+
+RUN cmake -S . -B build/linux-arm64 -G Ninja \
+        -DCMAKE_TOOLCHAIN_FILE=cmake/toolchains/clang-aarch64-linux-gnu.cmake \
+        -DCMAKE_BUILD_TYPE=Release \
+        -DOPENRA3_DISTRO_TAG=ubuntu26.04 \
+    && cmake --build build/linux-arm64 -j \
+    && (cd build/linux-arm64 && cpack -G DEB)
+
+# deploy target: runtime dependencies + final binary, minimal and fast. This is
+# an arm64 image; build it on (or with emulation for) an arm64 host.
+FROM --platform=linux/arm64 ubuntu:26.04 AS deploy
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+RUN --mount=type=cache,target=/var/lib/apt,sharing=locked \
+    --mount=type=cache,target=/var/cache/apt,sharing=locked \
+    apt-get -o Acquire::Retries=5 update \
+    &&     apt-get -o Acquire::Retries=5 install -y --no-install-recommends \
+        libc++1 \
+        libc++abi1 \
+        libsdl3-0 \
+        libvulkan1 \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /app
+COPY --from=dev /work/build/linux-arm64/bin/openra3 /app/openra3
+
+ENTRYPOINT ["/app/openra3"]