Fine-Grained Reactivity 2D: Observable Dependency Graph

Mobile apps that manage state with observables (MobX, Recoil-style atoms, SwiftUI's @State/@Published) don't re-render everything on every change — they track, per reaction, exactly which pieces of state that reaction actually read, and only re-run the reactions whose dependency set includes the atom that changed. This 2D simulator renders that dependency graph as two concentric rings: blue state atoms inside, orange reaction nodes outside, connected by the exact subscription edges a real observable store would build automatically. Mutate an atom and watch propagation happen live in either fine-grained mode (only true dependents flash green and re-run) or naive broadcast mode (every reaction re-runs regardless of whether it reads that atom), with live counters and a scrolling history strip quantifying exactly how many re-runs fine-grained tracking avoids.