Operator: Huber-Affine-Gate (Fused CUDA Kernel)

Definition
- z = x * scale + bias
- m = 0.5*z^2 if |z|<=1 else |z| - 0.5
- g = sigmoid(alpha * m + beta)
- y = x * g

Goal
- Piecewise smooth fusion; target ≥1.30x speedup.
