stalledFor method
Millis the sink has been stalled as of now, or 0 when healthy — a
backoff signal for the host scheduler (which owns cadence/backoff policy).
Implementation
int stalledFor(int now) {
final since = _stalledSince;
if (since == null) return 0;
final d = now - since;
return d > 0 ? d : 0;
}