Function project_mobility_ion_legacy 

pub fn project_mobility_ion_legacy(
    mean: f64,
    sigma: f64,
    scan_ids: &[u32],
    scan_mobilities: &[f64],
    im_cycle_length: f64,
    target_p: f64,
    step_size: f64,
) -> Vec<(i32, f64)>

LegacyCompat mobility projection for one ion: reproduce scan_occurrence/scan_abundance exactly as simulate_scan_distributions_with_variance did — calculate_scan_occurrence_ gaussian with n_lower/upper = 5, and calculate_abundance_gaussian over the fixed im_cycle_length bin. mean/sigma are the original stored 1/K0 mean + std at f64 (NOT a CCS round-trip). scan_mobilities must be ascending with scan_ids aligned (exactly as the legacy job feeds scans sorted so im_cycle_length > 0); a descending grid makes the occurrence kernel return nothing. The occurrence kernel’s indices are the values the legacy job stored directly, so they are returned as-is. Returns (scan, abundance).