Forced Duffing — dx/dt=y, dy/dt=−δ·y−α·x−β·x³+γ·cos(z), dz/dt=ω. Soft-spring classic δ=0.2 α=−1.0 β=1.0 γ=0.3 ω=1.0 (ω fixed). Dual γ+δ sliders. Display embed (x,y,sin z). Distinct from ueda.html (cubic, no linear restore), van-der-pol.html, colpitts.html, chua.html, lorenz.html. Bulk centroid ≈ (−0.0424, −0.0007, −0.0001). RK4 Duffing fold. Gold low-alpha trail, cap 650. RM = settled silhouette, no animate-on.