Function pio_bus_ids 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn pio_bus_ids(
    net: *const PioNetwork,
    out: *mut i64,
    cap: usize,
) -> usize

Write the 1-based external bus ids, in dense order, into out, up to cap entries, and return the total bus count. This ordering DEFINES the dense index space every other per-bus array shares. Call once with (NULL, 0) to size, allocate, then call again to fill. Ids are int64 in 1..2^63-1 (a v4 invariant); a source id that is a string or exceeds that range is mapped to dense int64 at read, never passed through raw.