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.

README.md

@@ -18,7 +18,7 @@ Ghidra.
 
 ## Status
 
-OpenRA3 is at **v0.4.0**. The engine compiles and runs headless, and a **minimal
+OpenRA3 is at **v0.5.0**. The engine compiles and runs headless, and a **minimal
 skirmish** is playable: it reads a real multiplayer map out of your install,
 recovers the player start waypoints, and simulates two sides building a base,
 extracting ore and fighting until one side is wiped out. The balance is the
@@ -122,13 +122,29 @@ their compilers and standard libraries never interfere.
 The host needs no toolchain: each target builds inside its own image.
 
 ```bash
-# Linux  -> build/linux/bin/openra3
+# Linux (amd64)   -> build/linux/bin/openra3 + .deb
 scripts/build-linux.sh
 
-# Windows -> build/windows/bin/openra3.exe (+ SDL3.dll)
+# Linux/arm64     -> build/linux-arm64/bin/openra3 + .deb   (cross)
+scripts/build-linux-arm64.sh
+
+# Debian (amd64)  -> build/debian/bin/openra3 + .deb
+scripts/build-debian.sh
+
+# Debian (arm64)  -> build/debian-arm64/bin/openra3 + .deb   (cross)
+scripts/build-debian-arm64.sh
+
+# Windows (x86_64) -> build/windows/bin/openra3.exe + SDL3.dll, .zip, .msi
 scripts/build-windows.sh
+
+# Windows (arm64)  -> build/windows-arm64/bin/openra3.exe + SDL3.dll, .zip   (cross)
+scripts/build-windows-arm64.sh
 ```
 
+Every target is cross-built from x86_64 Linux in its own image: llvm-mingw for
+the Windows targets, clang + libc++ with dpkg multiarch for the arm64 targets,
+and Debian/Ubuntu-specific images for the `.deb` packages.
+
 Or drive Docker directly:
 
 ```bash
@@ -160,6 +176,25 @@ openra3.exe render --game-dir "C:\Red Alert 3" --vulkan
 Vulkan is provided by **vendored volk + headers** (`third_party/`), resolved at
 runtime, so neither image needs a Vulkan SDK.
 
+## Packages
+
+CPack produces the release artifacts; the Docker images and CI run it for every
+target.
+
+| Target | Portable | Installer |
+| --- | --- | --- |
+| Linux amd64 (Ubuntu 26.04) | `.tar.gz` | `openra3_<v>_amd64ubuntu26.04.deb` |
+| Linux arm64 (Ubuntu 26.04) | `.tar.gz` | `openra3_<v>_arm64ubuntu26.04.deb` |
+| Linux amd64 (Debian 13) | `.tar.gz` | `openra3_<v>_amd64debian13.deb` |
+| Linux arm64 (Debian 13) | `.tar.gz` | `openra3_<v>_arm64debian13.deb` |
+| Windows x86_64 | `openra3-<v>-windows-x86_64.zip` | `openra3-<v>-windows-x86_64.msi` |
+| Windows arm64 | `openra3-<v>-windows-arm64.zip` | (WiX-only; see below) |
+
+The `.deb` dependencies are derived from the binary with `dpkg-shlibdeps`. The
+`.msi` is built with **wixl** (msitools) straight from the Linux cross image;
+wixl 0.106 has no arm64 support, so Windows/ARM64 ships the portable `.zip`
+(the MSI toolchain for ARM64 would be WiX v4 via the .NET SDK).
+
 ## Logging
 
 Every run writes `openra3.log` next to the executable through the vendored