Event-loop liveness contribution (wired into timers::drain_and_check’s
return): a pins pump whose last run reported live workers keeps the loop
alive. Worker pumps never contribute.
Drive every registered cluster pump function on the JS thread. Called from
timers::drain_and_check / drain_one_pass. Pump entries persist for the
realm’s lifetime (a primary pump that goes idle must still be present for
the next fork); only their last_alive liveness contribution follows the
return value.