Spherification: Reaction-Diffusion Gelation Profile

This is the 2D companion to the 3D spherification scene, and it computes gelation a genuinely different way. Instead of assuming a parabolic front shape, it discretizes a spherically-symmetric droplet onto a radial grid and integrates the coupled reaction-diffusion PDE for calcium-ion concentration and gel fraction directly, frame by frame, using an explicit finite-difference scheme (a version of the "shrinking-core" model used for gas/fluid–solid reactions in chemical engineering). The left panel renders the resulting concentric field as a cutaway disc plus a live grid of independently-gelling bath droplets; the right panel plots the actual radial concentration and gel-fraction curves the solver produced. A running comparison against the simple √(2Dt) diffusion estimate shows exactly how much slower the reaction-limited PDE front runs once Ca²⁺ consumption becomes the bottleneck — a correction the closed-form model can't see.