Function build_incidence 

pub fn build_incidence(
    case: &IndexedNetwork<'_>,
    conv: DcConvention,
) -> Result<IncidenceParts>

Build A, b, the phase shift injection, and the column→branch map.

Self-loops (from == to) and branches with x == 0 (no DC susceptance) are dropped, so m counts only the branches that contribute a column.