Per-activation hyperparameters for FlexNN::Activations.
Leaky ReLU with configurable alpha (default 0.01).
Linear (no) activation — identity forward/backward.
ReLU activation — max(0, z) forward, (z>0) backward.
Sigmoid activation — 1/(1+exp(-Z)) with clamping.
Stable column-wise softmax.
Tanh activation — std::tanh forward, (1-A^2) backward.