eventTimestamp

When the app fired this event, as an ISO-8601 UTC instant. Captured at the track/screen/identify call site (on the caller's thread), so it is independent both of the serial dispatcher's own stamping lag and of whatever event-time field the transport ends up stamping — either of which can lag behind by however long they batch/enqueue before sending. (Transports that stamp inside their own pipeline record their processing time here instead, since the app's call time isn't available to them.) Precision matches the platform clock backing kotlin.time.Clock.System (millisecond resolution on the JVM and Android; may be finer on other targets), not a guaranteed nanosecond timestamp.