Function classify_layer 

pub fn classify_layer(
    hist: &Histogram,
    fingerprints: &[LayerFingerprint],
    basis: &str,
) -> Option<LayerVerdict>

Nearest fingerprint by Euclidean distance over bucket %. Confidence is (max_dist - chosen_dist) / max_dist, penalized by small total_fns (< 5 fns flattens confidence regardless of fit).