Function fold_crossbar_for 

pub fn fold_crossbar_for(
    netlist: &AnalogNetlist,
    max_fanin: usize,
) -> AnalogNetlist

Fold to fixpoint, prune, then TREE the columns back down to the physical summing fan-in: a folded column wider than max_fanin splits into balanced sub-sums (weights ride on the leaves; the joining nodes take unit weights), because fan-in is a physical resource — the summing node’s loading, settling, and noise all grow with it.