feat(windows-client): cross-compile + ship ARM64 (aarch64) off the x64 runner

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>

scripts/ci/setup-windows-runner.ps1

@@ -84,6 +84,41 @@ if (-not (Test-Path "C:\Users\Public\.rustup\settings.toml")) {
     robocopy "$env:USERPROFILE\.rustup" "C:\Users\Public\.rustup" /E /NFL /NDL /NJH /NJS /MT:16 | Out-Null
 }
 
+# --- ARM64 cross-compile support (windows.yml / windows-msix.yml build aarch64-pc-windows-msvc off
+# this x64 box; no ARM64 runner needed). Two pieces beyond the x64 toolchain:
+#   1. the rustup std for the target;
+#   2. an ARM64 FFmpeg import-lib/DLL tree at C:\Users\Public\ffmpeg-arm64 (the workflow matrix
+#      points FFMPEG_DIR here for the aarch64 leg; the x64 tree stays at C:\Users\Public\ffmpeg).
+# The x64 MSVC toolset already ships the ARM64 cross compiler — if
+# VC\Tools\MSVC\<ver>\bin\Hostx64\arm64\cl.exe is missing, add the VS "MSVC v143+ ARM64/ARM64EC
+# build tools" + "C++ ARM64 build tools" workload components (the cc/cmake crates need it to
+# cross-build SDL3 + libopus).
+$env:RUSTUP_HOME = "C:\Users\Public\.rustup"
+$env:CARGO_HOME = "C:\Users\Public\.cargo"
+$rustup = (Get-Command rustup -ErrorAction SilentlyContinue).Source
+if (-not $rustup) { $rustup = "C:\Users\Public\.cargo\bin\rustup.exe" }
+if (Test-Path $rustup) {
+    Write-Host "==> rustup target add aarch64-pc-windows-msvc"
+    & $rustup target add aarch64-pc-windows-msvc
+} else { Write-Warning "rustup not found - install rustup then re-run (needed for the aarch64 target)." }
+
+$ffArm = "C:\Users\Public\ffmpeg-arm64"
+if (-not (Test-Path (Join-Path $ffArm 'lib\avcodec.lib'))) {
+    # BtbN winarm64 shared, FFmpeg 7.x (avcodec-61) to match the x64 tree's ABI. MSVC-linkable .lib
+    # import libs + headers + bin\*.dll — exactly what ffmpeg-sys-next + pack-msix.ps1 consume.
+    Write-Host "==> fetching ARM64 FFmpeg (BtbN winarm64 shared)"
+    $ffUrl = 'https://github.com/BtbN/FFmpeg-Builds/releases/download/latest/ffmpeg-n7.1-latest-winarm64-gpl-shared-7.1.zip'
+    $ffZip = "C:\Users\Public\ffmpeg-arm64.zip"
+    $ffTmp = "C:\Users\Public\ffmpeg-arm64-extract"
+    Invoke-WebRequest -Uri $ffUrl -OutFile $ffZip -UseBasicParsing
+    if (Test-Path $ffTmp) { Remove-Item -Recurse -Force $ffTmp }
+    Expand-Archive -Path $ffZip -DestinationPath $ffTmp -Force   # BtbN zips have one top-level folder
+    $inner = Get-ChildItem $ffTmp -Directory | Select-Object -First 1
+    if (Test-Path $ffArm) { Remove-Item -Recurse -Force $ffArm }
+    Move-Item -Path $inner.FullName -Destination $ffArm
+    Remove-Item -Force $ffZip; Remove-Item -Recurse -Force $ffTmp -ErrorAction SilentlyContinue
+}
+
 # Inno Setup (ISCC.exe) for the host installer build (windows-host.yml). pack-host-installer.ps1
 # locates it at its fixed Program Files path, so it need not be on PATH — just present.
 if (-not (Test-Path "C:\Program Files (x86)\Inno Setup 6\ISCC.exe")) {