Function classify_node 

pub fn classify_node(
    target: &Subject<'_>,
    idx: NodeIdx,
    rows: u64,
    used: &[Usage<'_>],
    fams: &Families,
) -> Result<(Option<Finding>, Vec<Finding>), VennError>

2026-09-29: The primary finding of target node idx (None: novel) and the secondary opportunities.