Cubic VdP — ẋ=y, ẏ=a(1−x²)y−x−0.2 x³+0.35 z, ż=−0.55 z−0.45 x. Classic a=0.5. Distinct from van-der-pol-rayleigh.html. RK4. Gold trail, cap 650. RM = settled silhouette, no animate-on.