Function pio_bus_demand 

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

Write the per-bus demand aggregates (active pd, reactive qd, summed over each bus’s loads, dense pio_bus_ids order), each up to cap entries, and return the total bus count. Either pointer may be NULL.