Function pio_summary_json 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn pio_summary_json(
    net: *const PioNetwork,
    errbuf: *mut c_char,
    errlen: usize,
) -> *mut c_char

Serialize a compact balanced network summary as JSON for display and scalar queries without serializing pio_to_json’s full payload.

counts is the case file’s own inventory, so it counts a 3-winding transformer once under transformers_3w rather than as the star bus and three branches it lowers to. topology.n_buses and topology.n_branches are that lowered space, the one pio_n_buses and pio_branches report and the one the rest of topology is computed over. The two differ only for a case with an in-service 3-winding transformer.