Function project_mobility_legacy_par 

pub fn project_mobility_legacy_par(
    means: &[f64],
    sigmas: &[f64],
    scan_ids: &[u32],
    scan_mobilities: &[f64],
    im_cycle_length: f64,
    target_p: f64,
    step_size: f64,
    num_threads: usize,
) -> Vec<Vec<(i32, f64)>>

Batched + parallel LegacyCompat scan projection over many ions (the kernels the legacy job used). means/sigmas are the original per-ion 1/K0 mean+std; scan_ids/scan_mobilities ascending+aligned (shared by all ions). Returns one (scan, abundance) list per ion. Requires positive sigmas (real data); use project_mobility_ion_legacy for the point-mass-guarded single- ion path.