Function field_mapping_direct 

fn field_mapping_direct(
    phases: &[&[f64]],
    mag_slices: &[&[f64]],
    mask: &[u8],
    metadata: &ScanMetadata,
    config: &FieldMappingConfig,
    _n_voxels: usize,
    nx: usize,
    ny: usize,
    nz: usize,
    vsx: f64,
    vsy: f64,
    vsz: f64,
    progress: &mut dyn FnMut(usize, usize),
) -> Result<FieldMappingResult, PipelineError>

Path B: Multi-echo without phase offset removal (per-echo unwrap + linear fit)