A sun-flared and bong-addled tumble into a teenage Texan summer rife with bombshells and boyfriend problems, “Cusp,” from debut directors Parker Hill and Isabel Bethencourt is one of those fractal-style documentaries, in which any given sliver contains all the colors and contours of the whole. The opening is a case in point: Long-haired girls lounge on a swing in the park, scoffing, wriggling, idly shooting the shit – it could be any year from any of the last five or six decades, except for the phones they glance at every now and then.