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windows.yml + windows-msix.yml gain an x86_64/aarch64 target matrix. ARM64 is cross-compiled on the one x64 Windows runner — the x64 MSVC toolset ships the ARM64 cross compiler, aarch64-pc-windows-msvc is tier-2 with host tools, and SDL3/libopus (build-from-source) cross-compile cleanly. The only arch-specific external dep is FFmpeg's import libs: the matrix points FFMPEG_DIR at a per-arch tree (x64 C:\Users\Public\ffmpeg, arm64 C:\Users\Public\ffmpeg-arm64, both FFmpeg 7.x / avcodec-61). Per-arch short CARGO_TARGET_DIR avoids a shared target dir; fmt + test run only for x64 (aarch64 can't execute on the x64 host). pack-msix.ps1 gains -Arch x64|arm64 (stamps the manifest ProcessorArchitecture, arch-suffixes the .msix/.cer); windows-msix.yml matrixes both arches and publishes ..._x64.msix / ..._arm64.msix. setup-windows-runner.ps1 provisions the rustup target + the ARM64 FFmpeg tree (idempotent). Verified live on the runner (home-windows-1): debug+release cross-build green, clippy -D warnings green, and MSIX pack produces a valid arm64 package (manifest arch=arm64; bundled exe/SDL3/avcodec/reactor-bootstrap all PE machine 0xAA64). Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>