progressOf method
Current progress toward questId's target, 0 if never registered
or if the stored progress belongs to a period that has since rolled
over (never throws).
Implementation
int progressOf(String questId) {
final def = _defs[questId];
if (def == null) return 0;
final record = _records[questId];
final periodKey = _currentPeriodKey(def.period);
if (record == null || record.periodKey != periodKey) return 0;
return record.progress;
}