Purge Started At

startedAt on a PurgeLog anchors the beginning of the purge event. Combined with endedAt, it establishes the total purge duration. Compliance staff use startedAt alongside measuredAt on the parent FieldMeasurement to verify that the purge was completed before the final field readings were recorded. Purge duration is also used with pumpRateGpm to validate the computed totalVolumeGallons.