Function core_invoice_convert 

#[unsafe(no_mangle)]
pub extern "C" fn core_invoice_convert(
    xml: *const c_char,
    to: *const c_char,
    profile: *const c_char,
    out: *mut c_char,
    out_len: usize,
    err: *mut c_char,
    err_len: usize,
) -> c_int

Convert through the semantic model. to is ubl or cii. On success writes XML into out (NUL-terminated). Same 0/1/2 as CLI convert.