FFI format 5 accepts retained, context-bearing `void` callbacks invoked by library-owned threads.
The native callback copies arguments and queues work; the script closure runs on the event loop at least one turn later.
These callbacks are not suitable for real-time execution.

Value-returning callbacks and direct script execution on foreign threads are unsupported.
Reference counting and exception state belong to the script thread, and waiting for the event loop can deadlock.