It started like these gatherings often do—unplanned, fast-moving, and already too large by the time authorities caught wind of it. By early Saturday evening, hundreds of teenagers had converged on Tybee Island’s pier and pavilion area, turning a typically quiet coastal spot into something closer to a flashpoint. Police described it as an “unpermitted, pop-up event,” the kind that spreads quickly through social media and brings in crowds with no …