The organizers of Saturday’s Virgin Mobile Free Festival saved the best for last. LCD Soundsystem — manifested onstage by brains-and-vox-of the outfit James Murphy-plus-six — closed down the sprawling 11-hour, three-stage, one-ferris wheel, no-T.I. music bazaar with an 85-minute blast of sweaty 21st-century dance-rock that felt taut and commanding after a long, discursive day. All dance tunes necessarily have a pulse, but Murphy’s have a heart, and they’re also wickedly funny. (Murphy was 22 when he turned down a job writing for Seinfeld. True!)