VdP–Rayleigh — ẋ=y, ẏ=−x+a(1−x²)y+0.4 z, ż=−0.6 z−0.3 x. Classic a=0.5. Distinct from van-der-pol-duffing.html / rayleigh-vdp.html. RK4. Gold trail, cap 650. RM = settled silhouette, no animate-on.