fix: make packaged updater handshake Windows-safe

scripts/apply-binary-update.ps1

@@ -7,7 +7,8 @@ param(
   [Parameter(Mandatory = $true)][int]$ParentPid,
   [Parameter(Mandatory = $true)][string]$LogPath,
   [Parameter(Mandatory = $true)][string]$StatusPath,
-  [Parameter(Mandatory = $true)][string]$UpdateId
+  [Parameter(Mandatory = $true)][string]$UpdateId,
+  [switch]$HandshakeOnly
 )
 
 $ErrorActionPreference = "Stop"
@@ -27,12 +28,26 @@ function Write-UpdateState {
     updatedAt = [DateTime]::UtcNow.ToString("o")
   }
   if ($State -in @("success", "failed", "rolled-back")) { $payload.completedAt = [DateTime]::UtcNow.ToString("o") }
+  $directory = Split-Path -Parent $StatusPath
+  if ($directory) { New-Item -ItemType Directory -Force -Path $directory | Out-Null }
   $temporary = "$StatusPath.$PID.tmp"
-  $payload | ConvertTo-Json -Depth 4 | Set-Content -LiteralPath $temporary -Encoding UTF8
-  if (Test-Path -LiteralPath $StatusPath) { [IO.File]::Replace($temporary, $StatusPath, $null) }
-  else { Move-Item -LiteralPath $temporary -Destination $StatusPath }
+  $backup = "$StatusPath.$PID.bak"
+  $json = $payload | ConvertTo-Json -Depth 4
+  $utf8NoBom = New-Object System.Text.UTF8Encoding($false)
+  [IO.File]::WriteAllText($temporary, $json, $utf8NoBom)
+  try {
+    if ([IO.File]::Exists($StatusPath)) {
+      [IO.File]::Replace($temporary, $StatusPath, $backup)
+      [IO.File]::Delete($backup)
+    } else {
+      [IO.File]::Move($temporary, $StatusPath)
+    }
+  } catch {
+    [IO.File]::Copy($temporary, $StatusPath, $true)
+    [IO.File]::Delete($temporary)
+    if ([IO.File]::Exists($backup)) { [IO.File]::Delete($backup) }
+  }
 }
-
 function Write-Log([string]$Message) {
   "{0} {1}" -f [DateTime]::UtcNow.ToString("o"), $Message | Add-Content -LiteralPath $LogPath -Encoding UTF8
 }
@@ -40,6 +55,10 @@ function Write-Log([string]$Message) {
 try {
   Write-UpdateState -State "started" -Message "Binary updater owns the update request."
   Write-Log "Validating ForgeFlow $ExpectedVersion binary update."
+  if ($HandshakeOnly) {
+    Write-Log "Handshake-only verification completed successfully."
+    exit 0
+  }
   $actualSha256 = (Get-FileHash -LiteralPath $BinaryPath -Algorithm SHA256).Hash.ToLowerInvariant()
   if ($actualSha256 -ne $ExpectedSha256.ToLowerInvariant()) { throw "Binary update SHA-256 verification failed." }
   if (-not (Test-Path -LiteralPath $CurrentExecutable -PathType Leaf)) { throw "Current ForgeFlow executable was not found." }