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>

CLAUDE.md

@@ -140,7 +140,11 @@ Low-latency desktop/game streaming stack, Linux-first, with a shared Rust protoc
    dev VM is headless/WARP; needs the RTX box.)** **Stream input** is Win32 low-level hooks (`WH_KEYBOARD_LL`/`WH_MOUSE_LL`) — reactor
    exposes no raw key/pointer events; native Windows VK + absolute mouse (client-rect Contain-fit) +
    wheel, Ctrl+Alt+Shift+Q capture toggle. `--headless`/`--discover` keep CLI paths. Builds + clippy
-   + fmt green on `x86_64-pc-windows-msvc` (on the dev VM). **windows-reactor is unpublished** (git
+   + fmt green on **`x86_64-pc-windows-msvc` and `aarch64-pc-windows-msvc`** — the latter
+   **cross-compiled off the one x64 runner** (no ARM64 runner; the x64 MSVC toolset's ARM64 cross
+   compiler + a per-arch `FFMPEG_DIR` ARM64 tree, SDL3/libopus build-from-source cross-compile
+   cleanly), and both ship as signed MSIX (`windows-msix.yml` matrix → `..._x64.msix`/`..._arm64.msix`,
+   verified: ARM64 binaries + manifest arch). **windows-reactor is unpublished** (git
    dep pinned to commit `b4129fcc`; `windows` pinned to the SAME commit so `IDXGISwapChain1` unifies
    with `set_swap_chain`); its `build.rs` downloads the Win App SDK NuGets + needs `CARGO_WORKSPACE_DIR`
    set (in the VM build env; `/temp`+`/winmd` gitignored). Gotcha: `CARGO_HOME` must be an ASCII path