Add Windows ARM64 build CI (#21817)

## Summary

Adds compile-only Windows ARM64 coverage to the existing MSVC workflow
using the official `windows-11-arm` hosted runner. The build targets
ARM64 with Visual Studio 2022 and covers the executor runner,
extensions, optimized kernels, XNNPACK, and LLM support. It follows the
existing Windows workflow run-decision and path-filtering policy.

## Test plan

The workflow passes YAML parsing, `git diff --check`, and actionlint
validation. Actionlint’s stale hosted-runner label list does not yet
include the official `windows-11-arm` label, so that specific warning
was excluded. The native Windows ARM64 compile will run on this PR.

This PR was authored with AI assistance from Codex.

Author: JacobSzwejbka

.ci/scripts/setup-windows-arm64.ps1

@@ -0,0 +1,48 @@
+Set-PSDebug -Trace 1
+$ErrorActionPreference = 'Stop'
+$PSNativeCommandUseErrorActionPreference = $true
+
+$vsWhere = "${env:ProgramFiles(x86)}\Microsoft Visual Studio\Installer\vswhere.exe"
+$vsInstallPath = & $vsWhere -latest -products * `
+    -requires Microsoft.VisualStudio.Component.VC.Tools.ARM64 `
+    -property installationPath
+if (-not $vsInstallPath) {
+    throw "Visual Studio with the ARM64 C++ toolchain was not found."
+}
+$vsDevShell = Join-Path $vsInstallPath "Common7\Tools\Launch-VsDevShell.ps1"
+& $vsDevShell -Arch arm64 -HostArch amd64
+
+$buildDir = "cmake-out-windows-arm64"
+if (Test-Path -Path $buildDir) {
+    Remove-Item -Path $buildDir -Recurse -Force
+}
+
+# XNNPACK's optional ARM ISA and assembly microkernels do not build with MSVC.
+# Baseline ARM64 NEON kernels remain enabled.
+cmake -S . -B $buildDir `
+    -G "Visual Studio 17 2022" `
+    -A ARM64 `
+    -DCMAKE_BUILD_TYPE=Release `
+    -DCMAKE_CXX_STANDARD=20 `
+    -DEXECUTORCH_BUILD_EXECUTOR_RUNNER=ON `
+    -DEXECUTORCH_BUILD_EXTENSION_DATA_LOADER=ON `
+    -DEXECUTORCH_BUILD_EXTENSION_MODULE=ON `
+    -DEXECUTORCH_BUILD_EXTENSION_TENSOR=ON `
+    -DEXECUTORCH_BUILD_EXTENSION_FLAT_TENSOR=ON `
+    -DEXECUTORCH_BUILD_EXTENSION_NAMED_DATA_MAP=ON `
+    -DEXECUTORCH_BUILD_KERNELS_OPTIMIZED=ON `
+    -DEXECUTORCH_BUILD_KERNELS_CUSTOM=OFF `
+    -DEXECUTORCH_BUILD_KERNELS_CUSTOM_AOT=OFF `
+    -DEXECUTORCH_BUILD_KERNELS_QUANTIZED=OFF `
+    -DEXECUTORCH_BUILD_XNNPACK=ON `
+    -DEXECUTORCH_BUILD_EXTENSION_LLM=ON `
+    -DEXECUTORCH_BUILD_EXTENSION_LLM_RUNNER=ON `
+    -DXNNPACK_ENABLE_ASSEMBLY=OFF `
+    -DXNNPACK_ENABLE_ARM_BF16=OFF `
+    -DXNNPACK_ENABLE_ARM_DOTPROD=OFF `
+    -DXNNPACK_ENABLE_ARM_FP16_SCALAR=OFF `
+    -DXNNPACK_ENABLE_ARM_FP16_VECTOR=OFF
+
+cmake --build $buildDir --config Release -j $env:NUMBER_OF_PROCESSORS
+
+Write-Host "Windows ARM64 build completed successfully!"

.github/workflows/windows-msvc.yml

@@ -11,6 +11,7 @@ on:
     paths:
       - .ci/docker/ci_commit_pins/pytorch.txt
       - .ci/scripts/**
+      - .github/workflows/windows-msvc.yml
   workflow_dispatch:
 
 concurrency:
@@ -57,3 +58,41 @@ jobs:
           \$PSNativeCommandUseErrorActionPreference = \$true
           .ci/scripts/setup-windows-msvc.ps1
         }"
+
+  build-windows-arm64:
+    name: build-windows-arm64
+    needs: [changed-files, run-decision]
+    if: |
+      contains(needs.changed-files.outputs.changed-files, '.ci/docker/ci_commit_pins/pytorch.txt') ||
+      contains(needs.changed-files.outputs.changed-files, '.ci/scripts/') ||
+      contains(needs.changed-files.outputs.changed-files, '.github/workflows/windows-msvc.yml') ||
+      needs.run-decision.outputs.is-full-run == 'true'
+    # test-infra's Windows setup assumes the AWS runner image; use the GitHub
+    # hosted Windows ARM64 image directly.
+    runs-on: windows-11-arm
+    timeout-minutes: 60
+    steps:
+      - name: Enable long paths
+        shell: cmd
+        run: |
+          git config --system --get core.longpaths || echo "core.longpaths is not set, setting it now"
+          git config --system core.longpaths true
+
+      - name: Checkout ExecuTorch
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
+
+      - name: Initialize submodules
+        shell: pwsh
+        run: |
+          git config --global http.sslBackend openssl
+          git submodule update --init --recursive
+
+      - name: Install build dependencies
+        shell: pwsh
+        run: python -m pip install pyyaml torch==2.13.0 --extra-index-url https://download.pytorch.org/whl/test/cpu
+
+      - name: Build ExecuTorch
+        shell: pwsh
+        run: .ci/scripts/setup-windows-arm64.ps1