2D Fireworks: Real Projectile & Burst Physics

This is a genuine two-stage physics simulation, not a canned particle-fade effect. Click anywhere in the sky and a shell launches from the ground with an initial upward velocity solved from real projectile-motion equations so it reaches its apex near your click; gravity decelerates it every frame exactly as it would a thrown ball. The instant the shell's vertical velocity crosses zero — true apex, detected from the physics itself rather than a timer — it detonates into dozens or hundreds of independent spark particles, each integrated frame by frame under gravity and air drag. Four shell types (peony, chrysanthemum, willow, crossette) change the actual burst parameters — spark count, launch speed, drag coefficient, and for crossette a genuine secondary in-flight burst — rather than only recolouring the same dots.