rontolisp:threadp
(rontolisp:threadp value)
Returns t when value is a thread handle (as returned by
rontolisp:make-thread), else nil. This is the one
thread operation that accepts any value.
On the WASM backends no thread handle can exist, so the bt2:threadp shim constantly
answers nil there — which is how Clack's stop takes its non-threaded branch.