fix(template): NSIS download browser User-Agent + PE magic bytes guard

template/Setup-WinBuild2025.ps1

@@ -1199,8 +1199,21 @@ if (Test-Path $nsisExe) {
     $nsisUrl       = "https://prdownloads.sourceforge.net/nsis/nsis-$NSISVersion-setup.exe"
     $nsisInstaller = 'C:\CI\nsis_installer.exe'
     Write-Host "Downloading NSIS $NSISVersion from SourceForge..."
-    try { Invoke-WebRequest -Uri $nsisUrl -OutFile $nsisInstaller -UseBasicParsing }
-    catch { throw "Failed to download NSIS from '$nsisUrl': $_" }
+    # SourceForge returns an HTML redirect page when accessed with PS default User-Agent.
+    # Supplying a browser-like User-Agent forces the CDN to serve the raw binary.
+    try {
+        Invoke-WebRequest -Uri $nsisUrl -OutFile $nsisInstaller -UseBasicParsing `
+            -Headers @{ 'User-Agent' = 'Mozilla/5.0 (Windows NT 10.0; Win64; x64)' }
+    } catch { throw "Failed to download NSIS from '$nsisUrl': $_" }
+    # Guard: verify downloaded file is a valid PE executable (MZ magic bytes).
+    # If SourceForge still returned an HTML page the installer would silently corrupt
+    # the WinRM session — fail fast with a clear message instead.
+    $peBytes = [System.IO.File]::ReadAllBytes($nsisInstaller)
+    if ($peBytes.Length -lt 2 -or $peBytes[0] -ne 0x4D -or $peBytes[1] -ne 0x5A) {
+        Remove-Item $nsisInstaller -ErrorAction SilentlyContinue
+        throw "NSIS download is not a valid PE executable (MZ header missing). " +
+              "SourceForge likely returned an HTML page. Set `$script:Hashes['NSIS']` to pin a known-good hash."
+    }
     Assert-Hash -FilePath $nsisInstaller -Label "nsis-$NSISVersion-setup.exe" `
                 -Expected $script:Hashes['NSIS']
     Write-Host "Installing NSIS $NSISVersion (silent /S)..."