fix(ci): ISO-assert discards stale WIM mount + asserts app catalog baked

Assert-IsoStructure.ps1 reused a fixed mount dir; a prior aborted run left a WIM
mounted there, so Mount-WindowsImage failed with 'directory is not empty' and the
persist-to-stable-path step was skipped (no ISO deployed). Now discards stale mounts
+ clears corrupt mount points + uses a unique per-run mount dir (mirrors build.ps1
Stage 0), and removes the dir after. Also asserts apps/catalog.json baked into the WIM.

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

windows/tests/Assert-IsoStructure.ps1

@@ -17,8 +17,16 @@ function Assert { param([string]$Name,[bool]$Cond)
 if (-not (([Security.Principal.WindowsPrincipal][Security.Principal.WindowsIdentity]::GetCurrent()).IsInRole(
     [Security.Principal.WindowsBuiltInRole]::Administrator))) { throw 'must run elevated (WIM mount).' }
 
+# Discard any stale WIM mounts from a prior aborted run (CI reuses the runner) and
+# use a fresh, unique mount dir — a leftover mount makes Mount-WindowsImage fail with
+# "attempted to mount to a directory that is not empty." Mirrors build.ps1 Stage 0.
+Get-WindowsImage -Mounted -EA SilentlyContinue | ForEach-Object {
+    Dismount-WindowsImage -Path $_.MountPath -Discard -EA SilentlyContinue | Out-Null
+}
+Clear-WindowsCorruptMountPoint -EA SilentlyContinue | Out-Null
+
 $img = Mount-DiskImage -ImagePath $IsoPath -PassThru
-$mount = Join-Path $env:TEMP 'sm-assert-wim'
+$mount = Join-Path $env:TEMP ('sm-assert-wim-' + [guid]::NewGuid().ToString('N'))
 $null = New-Item -ItemType Directory -Force $mount
 try {
     $drive = ($img | Get-Volume).DriveLetter + ':'
@@ -43,10 +51,15 @@ try {
                 Assert 'Welcome exe baked into WIM' (Test-Path $welcomeExe)
                 $welcomeFlavours = Get-ChildItem (Join-Path $mount 'Program Files\SilverOS\Welcome\flavours') -Filter '*.json' -EA SilentlyContinue
                 Assert 'Welcome flavours baked (>=1 .json)' ($welcomeFlavours.Count -ge 1)
+                $welcomeCatalog = Join-Path $mount 'Program Files\SilverOS\Welcome\apps\catalog.json'
+                Assert 'Welcome app catalog baked' (Test-Path $welcomeCatalog)
             }
         } finally { Dismount-WindowsImage -Path $mount -Discard | Out-Null }
     }
-} finally { Dismount-DiskImage -ImagePath $IsoPath | Out-Null }
+} finally {
+    Dismount-DiskImage -ImagePath $IsoPath | Out-Null
+    Remove-Item $mount -Recurse -Force -EA SilentlyContinue
+}
 
 Write-Host "`n$($fail) assertion(s) failed."
 exit $fail