🧪 Jellyfish Bloom (2D): Bell-Pulse Jet Propulsion Physics

Real jellyfish move by jet propulsion: the bell contracts, its cross-section shrinks, and the water it expels pushes the animal upward for a moment before the bell relaxes and the cycle repeats. This 2D companion turns that mechanic into a readable side view instead of an orbiting 3D scene — each medusa's bell radius follows R(t) = R₀·(0.82 + 0.28·sin(φ)), and whenever that radius is shrinking (bell contracting) the simulation derives a thrust force from the rate of change of the bell's cross-sectional area, exactly the physical mechanism real medusae rely on. That thrust fights a constant downward sink bias, a velocity-proportional drag term, and a slowly wandering horizontal current, so the population drifts and rises in a way that stays readable at a glance: turn up "Thrust strength" and the bloom visibly out-climbs its own sinking, turn up "Current strength" and the whole bloom leans sideways.