Duffing–Rayleigh — ẋ=y, ẏ=−0.2 y+x−x³+a(1−y²)y+0.3 cos z, ż=1. Classic a=0.3. Phase-wrap z. Distinct from duffing-holmes.html / rayleigh-vanderpol.html. RK4. Gold trail, cap 650. RM = settled silhouette, no animate-on.