Private ReadonlyrealReal (worker-side) in-flight task count per worker; absent means 0.
Current real in-flight count for worker (0 when none).
Record a dispatch of task to target, chosen from the live workers.
task must be the worker call's own promise (not a timeout/abort race),
so the count drops only when the worker has actually finished.
Privatehas
Counts worker-side in-flight tasks and records dispatch performance counters.