Function process_single_file 

pub fn process_single_file(
    input_path: impl AsRef<Path>,
    target_lufs: f64,
    target_peak_db: f64,
    input_base_dir: impl AsRef<Path>,
    output_base_dir: &Option<impl AsRef<Path>>,
    cache: &Option<HashMap<PathBuf, Result<f64, MeasurementError>>>,
) -> Result<(), Error>

Main orchestration function for processing a single file using the two-pass method.