fix: persistence.dat Method B now uses docker-desktop WSL for ext4 formatting

Previously, when no standard WSL distro was available, Method B only created
an empty sparse file without ext4 formatting. This caused Ventoy to fail
mounting persistence.dat on boot, dropping to initramfs.

Now Method B tries docker-desktop WSL (/sbin/mkfs.ext4) before falling back
to raw file. Also added ext4 magic number (0xEF53) verification step.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

bootable/3-create-persistence.ps1

@@ -93,12 +93,8 @@ if ($wslDistro) {
     }
 
 } else {
-    # ── Method B: Create raw file with PowerShell, format in Linux later ──
-    Write-Host "[INFO] No usable WSL distro (only docker-desktop found or none)." -ForegroundColor Yellow
-    Write-Host ""
-    Write-Host "  Will create a raw persistence image file." -ForegroundColor White
-    Write-Host "  It will be formatted automatically on first Linux boot." -ForegroundColor White
-    Write-Host ""
+    # ── Method B: Try docker-desktop WSL, fallback to raw file ──
+    Write-Host "[INFO] No standard WSL distro found." -ForegroundColor Yellow
 
     $sizeInput = Read-Host "Persistence size in GB (default: $DefaultSizeGB for 32GB USB)"
     if ([string]::IsNullOrWhiteSpace($sizeInput)) {
@@ -113,20 +109,67 @@ if ($wslDistro) {
         exit 1
     }
 
-    Write-Host "[INFO] Creating ${sizeGB} GB sparse file (fast, only allocates on write)..." -ForegroundColor Yellow
+    $sizeMB = $sizeGB * 1024
+    $formatted = $false
 
-    # Create sparse file using .NET (instant, doesn't write zeros)
-    $sizeBytes = [int64]$sizeGB * 1024 * 1024 * 1024
-    $fs = [System.IO.File]::Create($PersistencePath)
-    $fs.SetLength($sizeBytes)
-    $fs.Close()
+    # Try docker-desktop WSL (has /sbin/mkfs.ext4)
+    $dockerWSL = $null
+    try {
+        $allDistros = (wsl --list --quiet 2>$null) -replace "`0","" | Where-Object { $_.Trim() -ne "" -and $_ -match "docker" }
+        foreach ($d in $allDistros) {
+            $d = $d.Trim()
+            if ($d) {
+                $testResult = wsl -d $d -- /sbin/mkfs.ext4 -V 2>&1
+                if ($LASTEXITCODE -eq 0 -or "$testResult" -match "mke2fs") {
+                    $dockerWSL = $d
+                    break
+                }
+            }
+        }
+    } catch {}
+
+    if ($dockerWSL) {
+        Write-Host "[INFO] Found docker-desktop WSL with mkfs.ext4, creating ext4 image..." -ForegroundColor Green
+        $tmpFile = "/tmp/uclaw_persistence.dat"
+
+        # Create sparse file and format in docker-desktop WSL
+        wsl -d $dockerWSL -- dd if=/dev/zero of=$tmpFile bs=1M count=0 seek=$sizeMB 2>$null
+        wsl -d $dockerWSL -- /sbin/mkfs.ext4 -F -L casper-rw $tmpFile 2>$null
+
+        # Copy out via /mnt/host/c/ (docker-desktop mounts C: there)
+        $tmpHostPath = "C:\uclaw_persistence_tmp.dat"
+        wsl -d $dockerWSL -- cp $tmpFile /mnt/host/c/uclaw_persistence_tmp.dat 2>$null
+
+        if (Test-Path $tmpHostPath) {
+            Move-Item -Path $tmpHostPath -Destination $PersistencePath -Force
+            wsl -d $dockerWSL -- rm -f $tmpFile 2>$null
+            $formatted = $true
+            Write-Host "[OK]   ext4 image created via docker-desktop WSL." -ForegroundColor Green
+        } else {
+            Write-Host "[WARN] docker-desktop copy failed, falling back to raw file." -ForegroundColor Yellow
+            wsl -d $dockerWSL -- rm -f $tmpFile 2>$null
+        }
+    }
+
+    if (-not $formatted) {
+        # Fallback: create raw sparse file
+        Write-Host "[INFO] Creating ${sizeGB} GB sparse file (needs manual format in Linux)..." -ForegroundColor Yellow
+        $sizeBytes = [int64]$sizeGB * 1024 * 1024 * 1024
+        $fs = [System.IO.File]::Create($PersistencePath)
+        $fs.SetLength($sizeBytes)
+        $fs.Close()
+    }
 
     if (Test-Path $PersistencePath) {
-        Write-Host "[OK]   Created ${sizeGB} GB persistence file." -ForegroundColor Green
-        Write-Host ""
-        Write-Host "  IMPORTANT: After first boot into Linux, run this to format it:" -ForegroundColor Yellow
-        Write-Host '  sudo mkfs.ext4 -F -L casper-rw /media/*/Ventoy/persistence.dat' -ForegroundColor White
-        Write-Host '  Then reboot for persistence to take effect.' -ForegroundColor White
+        $actualSize = [math]::Round((Get-Item $PersistencePath).Length / 1GB, 1)
+        Write-Host "[OK]   Created ${actualSize} GB persistence file." -ForegroundColor Green
+        if (-not $formatted) {
+            Write-Host ""
+            Write-Host "  IMPORTANT: File is NOT yet formatted as ext4!" -ForegroundColor Yellow
+            Write-Host "  After first boot into Linux, run:" -ForegroundColor Yellow
+            Write-Host '  sudo mkfs.ext4 -F -L casper-rw /media/*/Ventoy/persistence.dat' -ForegroundColor White
+            Write-Host '  Then reboot for persistence to take effect.' -ForegroundColor White
+        }
     } else {
         Write-Host "[ERROR] Failed to create persistence file." -ForegroundColor Red
         Read-Host "Press Enter to exit"
@@ -134,6 +177,26 @@ if ($wslDistro) {
     }
 }
 
+# ── Verify ext4 magic number ──
+if (Test-Path $PersistencePath) {
+    try {
+        $f = [System.IO.File]::OpenRead($PersistencePath)
+        $f.Seek(1080, 0) | Out-Null
+        $buf = New-Object byte[] 2
+        $f.Read($buf, 0, 2) | Out-Null
+        $f.Close()
+        $magic = "{0:X2}{1:X2}" -f $buf[0], $buf[1]
+        if ($magic -eq "53EF") {
+            Write-Host "[OK]   ext4 verified (magic: 0xEF53)." -ForegroundColor Green
+        } else {
+            Write-Host "[WARN] NOT a valid ext4 filesystem (magic: 0x$magic)." -ForegroundColor Yellow
+            Write-Host "       You must format it in Linux before persistence will work." -ForegroundColor Yellow
+        }
+    } catch {
+        Write-Host "[WARN] Could not verify ext4 magic number." -ForegroundColor Yellow
+    }
+}
+
 Write-Host ""
 Write-Host "[OK]   Step 3 complete! Persistence image created." -ForegroundColor Green
 Write-Host "       Path: $PersistencePath" -ForegroundColor White