Pack: wrap the dist zip in a single BT412/ folder

The zip previously stored files at the root (Compress-Archive -Path dist\*),
so extracting spilled loose files into the extraction directory. Build the
archive with .NET's ZipFile and prefix every entry with BT412/, so it extracts
into one self-contained BT412\ folder. Local staging dir stays dist\.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pack-dist.ps1

@@ -253,8 +253,24 @@ Write-Host ("dist ready: {0:N1} MB  ->  $dist" -f ($size / 1MB))
 
 if ($Zip) {
     $zipPath = Join-Path $root 'BattleTech-4.12.zip'
-    Write-Host "zipping to $zipPath..."
+    Write-Host "zipping to $zipPath (wrapped in a BT412\ folder)..."
     if (Test-Path $zipPath) { Remove-Item -Force $zipPath }
-    Compress-Archive -Path "$dist\*" -DestinationPath $zipPath -Force
-    Write-Host "zip ready: $zipPath"
+    # Every entry is prefixed BT412/ so the archive extracts into ONE folder
+    # (rather than spilling loose files into the extraction directory).  Built
+    # with .NET's ZipFile so the prefix is explicit, independent of the local
+    # staging dir's name.
+    Add-Type -AssemblyName System.IO.Compression.FileSystem
+    $base = (Resolve-Path $dist).Path.TrimEnd('\')
+    $archive = [System.IO.Compression.ZipFile]::Open($zipPath, 'Create')
+    try {
+        Get-ChildItem -LiteralPath $dist -Recurse -File | ForEach-Object {
+            $rel = $_.FullName.Substring($base.Length + 1) -replace '\\', '/'
+            [System.IO.Compression.ZipFileExtensions]::CreateEntryFromFile(
+                $archive, $_.FullName, "BT412/$rel",
+                [System.IO.Compression.CompressionLevel]::Optimal) | Out-Null
+        }
+    } finally {
+        $archive.Dispose()
+    }
+    Write-Host "zip ready: $zipPath (extracts to a single BT412\ folder)"
 }