Function best_ms1_for_each_ms2_xic 

pub fn best_ms1_for_each_ms2_xic(
    ms1: &[ClusterResult1D],
    ms2: &[ClusterResult1D],
    pairs: &[(usize, usize)],
    opts: &XicScoreOpts,
) -> Vec<Option<usize>>

XIC-based assignment: score via RT/IM traces + intensity ratio, choose best MS1 per MS2, return Vec<Option<ms1_idx>> indexed by ms2_idx.