Function pio_gridfm_scenario_ids 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn pio_gridfm_scenario_ids(
    dir: *const c_char,
    out: *mut i64,
    cap: usize,
    errbuf: *mut c_char,
    errlen: usize,
) -> isize

Write a gridfm dataset’s distinct scenario ids (ascending) into out, up to cap entries, and return the total count. Call once with cap == 0 (or out NULL) to size, allocate, then call again to fill — the standard count/buffer pattern of pio_bus_ids. Returns -1 on error and writes the message into errbuf. Built --features gridfm.