Function pio_bus_shunt 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn pio_bus_shunt(
    net: *const PioNetwork,
    gs: *mut f64,
    bs: *mut f64,
    cap: usize,
) -> usize

Write the per-bus shunt aggregates (conductance gs, susceptance bs, dense pio_bus_ids order), each up to cap entries, and return the total bus count. Either pointer may be NULL.