fix(windows): clean up temporary qpdf staging files

[McanKul]

Goal

Prevent the Windows qpdf preparation script from leaving offpdf-qpdf-* staging directories in the system temp folder.

Acceptance criteria

• Temporary staging is removed after both successful and failed runs.
• Cleanup is guaranteed with try/finally.
• Copied qpdf binaries and Visual C++ runtime files remain intact.
• Existing script output and version handling remain unchanged.

Files

• scripts/prepare-qpdf-windows.ps1

Verification

• Run the script in PowerShell and compare %TEMP%\offpdf-qpdf-* before and after.
• Confirm the expected qpdf and runtime files are still staged.