OXNARD, Calif. — The third-team quarterback is guiding his unit down the field in team drills Sunday, closing in on the goal line but running out of time according to the end zone clock. A fan shouts, “Let’s go Trey, shake the rust off …’’ Rust, indeed. Trey Lance, the mystery acquisition the Dallas Cowboys made 11 months ago, is perhaps the mystery man of the entire NFL. The quarterback selected with the third pick of the 2021 draft by the San Francisco 49ers — does he still possess the potential that made Kyle Shanahan and John Lynch trade three first-round picks to move up to get him?