NASHVILLE, Tenn.—Mariah Bell put together a mesmerizing free skate at the U.S. Figure Skating Championships on Friday night, edging Karen Chen for her first gold medal and taking any drama out of whether she’s headed to her first Winter Olympics. Bell led after a spectacular short program, but it took an equally incredible performance to k.d. lang’s rendition of the Leonard Cohen staple “Hallelujah” to reach the top step of the podium. She earned 140.70 points in her free skate and had 216.25 in total, leaving Chen nearly three points back in second place. “I’m just really grateful to be here and be competing,” said the 25-year-old Bell, the oldest national champion in 95 years. “I was really nervous for this program and just tried my best to stay in the moment, grab every point, and the audience is so amazing and I’m just so honored and grateful.” Fourteen-year-old Isabeau …