Bell cubic — dx/dt=y, dy/dt=z, dz/dt=−a·z−b·y+c·x·(1−x²). Classic a=0.45 b=0.5 c=0.6. Dual a+c sliders; b fixed 0.5. Display embed (x,y,z). Distinct from oregonator.html, willamowski.html, duffing.html, sprott-jerk.html, lorenz.html. Bulk centroid ≈ (0.3403, 0.1607, 0.0971). RK4 cubic fold. Gold low-alpha trail, cap 650. RM = settled silhouette, no animate-on.