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.github/actions/build_companion/setup-msvc.ps1

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+# setup-msvc.ps1
+# Runs vcvarsall.bat and exports only the new/changed environment
+# variables to GitHub Actions, avoiding clobbering existing ones.
+
+param(
+    [string]$Arch = "x64"
+)
+
+# Find VS installation
+$vsWhere = "C:\Program Files (x86)\Microsoft Visual Studio\Installer\vswhere.exe"
+$vsPath  = & $vsWhere -latest -property installationPath
+$vcvars  = Join-Path $vsPath "VC\Auxiliary\Build\vcvarsall.bat"
+
+if (-not (Test-Path $vcvars)) {
+    Write-Error "vcvarsall.bat not found at: $vcvars"
+    exit 1
+}
+
+# Snapshot environment before
+$before = @{}
+Get-ChildItem env: | ForEach-Object { $before[$_.Name] = $_.Value }
+
+# Run vcvarsall.bat and import its environment into this process
+cmd /c "`"$vcvars`" $Arch && set" | ForEach-Object {
+    if ($_ -match '^([^=]+)=(.*)$') {
+        [System.Environment]::SetEnvironmentVariable($Matches[1], $Matches[2])
+    }
+}
+
+# Export only changed/new variables to GitHub Actions
+$count = 0
+Get-ChildItem env: | ForEach-Object {
+    if ($before[$_.Name] -ne $_.Value) {
+        "$($_.Name)=$($_.Value)" | Out-File -Append -Encoding utf8 $env:GITHUB_ENV
+        $count++
+    }
+}
+
+Write-Host "Exported $count environment variables to GITHUB_ENV"