ATLANTA — Jordan Spieth was so consumed with trying to make birdies and save pars on a rugged, rainy afternoon at the Tour Championship that he didn’t realize until after he signed for a 2-under 68 that he had a one-shot lead. He knows exactly what’s at stake Sunday in what was shaping up as a dynamic end to the season. Spieth made four big par saves and ended with a 20-foot birdie to overtake Henrik Stenson for the lead at East Lake.