fix: make updater checksum verification self-contained

scripts/apply-source-update.ps1

@@ -71,6 +71,17 @@ function Write-UpdateState {
   }
 }
 
+function Get-Sha256([string]$Path) {
+  $stream = [IO.File]::OpenRead($Path)
+  $algorithm = [Security.Cryptography.SHA256]::Create()
+  try {
+    return ([BitConverter]::ToString($algorithm.ComputeHash($stream))).Replace("-", "").ToLowerInvariant()
+  } finally {
+    $algorithm.Dispose()
+    $stream.Dispose()
+  }
+}
+
 function Invoke-Robocopy {
   param([string]$From, [string]$To)
   New-Item -ItemType Directory -Force -Path $To | Out-Null
@@ -120,7 +131,7 @@ try {
     Start-Sleep -Milliseconds 500
   }
 
-  $actualHash = (Get-FileHash -LiteralPath $ArchivePath -Algorithm SHA256).Hash.ToLowerInvariant()
+  $actualHash = Get-Sha256 -Path $ArchivePath
   if ($actualHash -ne $ExpectedSha256.ToLowerInvariant()) { throw "Update archive checksum mismatch." }
 
   $working = Join-Path ([IO.Path]::GetTempPath()) ("forgeflow-update-" + [guid]::NewGuid().ToString("N"))