Add Objective-C/Swift-friendly nullability annotations to include/ctap2.h without changing the C ABI.\n\nScope:\n- preserve existing exported function names and signatures semantically\n- document which pointers may be null, including allow_list_ids and keepalive callbacks\n- keep caller-owned buffers explicit\n- update generated C API docs after the header comments change.
Add Objective-C/Swift-friendly nullability annotations to include/ctap2.h without changing the C ABI.\n\nScope:\n- preserve existing exported function names and signatures semantically\n- document which pointers may be null, including allow_list_ids and keepalive callbacks\n- keep caller-owned buffers explicit\n- update generated C API docs after the header comments change.