Mathieu–Rayleigh — ẋ=y, ẏ=−(1+a cos z)x + 0.3(1−y²)y, ż=1. Phase-wrap z. Classic a=0.5. Distinct from mathieu-duffing.html / rayleigh-vanderpol.html. RK4. Gold trail, cap 650. RM = settled silhouette, no animate-on.