ci(windows): wire SignPath test-signing for portable bundle

Add SignPath artifact configuration (windows-portable-v1) signing only the
first-party binaries orca-slicer.exe and OrcaSlicer.dll, plus verification
and inventory scripts. The Windows build job uploads the unsigned portable
bundle, submits it to SignPath (test-signing), verifies the returned
signatures, and rebuilds the installer/zip from the signed binaries.

project-slug matches the SignPath project (OrcaSlicer).

scripts/inventory-authenticode.ps1

@@ -0,0 +1,48 @@
+[CmdletBinding()]
+param(
+    [Parameter(Mandatory = $true)]
+    [ValidateScript({ Test-Path -LiteralPath $_ -PathType Container })]
+    [string]$ArtifactDirectory
+)
+
+$ErrorActionPreference = "Stop"
+
+function Get-RelativePath {
+    param(
+        [Parameter(Mandatory = $true)]
+        [string]$Root,
+
+        [Parameter(Mandatory = $true)]
+        [string]$Path
+    )
+
+    $rootWithSeparator = $Root.TrimEnd([IO.Path]::DirectorySeparatorChar, [IO.Path]::AltDirectorySeparatorChar) + [IO.Path]::DirectorySeparatorChar
+    $rootUri = [Uri]$rootWithSeparator
+    $pathUri = [Uri]$Path
+
+    [Uri]::UnescapeDataString($rootUri.MakeRelativeUri($pathUri).ToString()).Replace("/", [IO.Path]::DirectorySeparatorChar)
+}
+
+$artifactRoot = (Resolve-Path -LiteralPath $ArtifactDirectory).Path
+$binaries = Get-ChildItem -LiteralPath $artifactRoot -Recurse -File |
+    Where-Object { $_.Extension -in @(".exe", ".dll") } |
+    Sort-Object -Property FullName
+
+if (-not $binaries) {
+    Write-Warning "No .exe or .dll files found under '$artifactRoot'."
+    exit 0
+}
+
+$binaries | ForEach-Object {
+    $signature = Get-AuthenticodeSignature -LiteralPath $_.FullName
+
+    [pscustomobject]@{
+        RelativePath = Get-RelativePath -Root $artifactRoot -Path $_.FullName
+        Status = $signature.Status
+        SignatureType = $signature.SignatureType
+        Signer = if ($signature.SignerCertificate) { $signature.SignerCertificate.Subject } else { "" }
+        SignerThumbprint = if ($signature.SignerCertificate) { $signature.SignerCertificate.Thumbprint } else { "" }
+        Timestamped = [bool]$signature.TimeStamperCertificate
+        TimeStamper = if ($signature.TimeStamperCertificate) { $signature.TimeStamperCertificate.Subject } else { "" }
+    }
+} | Format-Table -AutoSize