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. But then in the background, there’s a popping noise as one of the peripheral guys in their endlessly forming and reforming group whoops and shoots his gun for target practice. The girls scarcely bat an eyelid but there it all is, in that one scene: girlhood, unconsciously hopeful and lovely despite itself, and the darker edge of recklessness and violence – often springing from men and boys – to which they are already resigned.