The afternoon theme du jour is that rap goes with everything — no surprise, perhaps, when the night’s headliner is Jay-Z, who has become the man for all festivals. But the more improbable corollary is that rap-rock, the widely derided late-1990s fusion that dominated FM rock radio for a few tedious years, is recovering some of its early promise. The day’s first blast of it was by P.O.S., a Minnesota rapper who details setbacks, struggles, contradictions and small, hard-won successes: “I can never be what I want to be/Nothing but a man,” he rapped. His rhymes are fast, with jagged rhythms; his band pounds without steamrollering him, and he keeps his shoutalongs simple enough to keep the crowd with him: “Yeah right!” went one, “Yeah,” went another.