Function layer_violations 

pub fn layer_violations(
    graph: &Graph,
    layers: &[Layer],
    allowed: &HashSet<EdgeKind>,
) -> Vec<Violation>

Evaluate --layers: layers are ordered highest first — a layer may depend on layers listed after it, never before. One Violation per offending member of a lower layer that reaches a higher layer over allowed edges (so every violator is named, not just the first), each carrying its dependency path.