submit

Schedule code on this lane. It compiles off the tick, joins the lane once compiled, and is stepped every heartbeat until done. It's bound to the epoch current at submit: if the heartbeat has stopped for good by the time it finishes compiling, it's killed rather than queued forever. The returned EvalHandle carries the result future (callers block on it — block-until-done) and can cancel the eval (the client-initiated notifications/cancelled path).