One of the most beloved and influential institutions in the history of local independent music, Teenbeat Records marked its 26th anniversary on Saturday night with a concert at the Black Cat that showcased four of the best bands in its stable: Unrest, Versus, The Rondelles, and Bossanova. Founded by Mark Robinson and friends at Arlington’s Wakefield High School, the label is now based in Cambridge, Massachusetts — but its DIY ethos and sonic signatures remain…