Function compute_coupling_absences 

pub fn compute_coupling_absences<S: BuildHasher>(
    base_coupling: &[CouplingRow],
    pr_files: &HashSet<String, S>,
    min_shared: u32,
    fisher_p_gate: f64,
) -> Vec<CouplingAbsence>

Flag every coupling pair where exactly one side is in the changed set: the touched file’s historically-coupled partner is absent, which is the signal a co-change was forgotten. Advisory — callers decide severity.