Function on_the_front_end_stack 

pub fn on_the_front_end_stack<T: Send>(f: impl FnOnce() -> T + Send) -> T

Run f on a thread that has STACK_BYTES, and give back what it returned.

The counterpart of beck_eval::on_the_evaluator_stack, and the answer to “who guarantees the count is reachable” for the front end. A caller already inside on_the_evaluator_stack — which is every path through beck-cli — needs neither, because the two declare the same number.