Function scratch_file_path 

fn scratch_file_path(
    app: &AppHandle,
    session_id: u64,
) -> Result<PathBuf, String>

Scratch file path for session_id — scratch-<id>.mp4. The scratch is always MP4/H.264 (the canonical intermediate the Save panel moves or transcodes).

Errors

"app cache dir unavailable …" when the platform path resolver fails (no $HOME, sandbox without a cache dir).