feat: add clamp/clip autograd gate for gradient clipping

[ulises-jeremias]

Training stability requires gradient clipping, but there's no clamp/clip gate
in the autograd engine. The forward pass is min(max(x, lo), hi); gradient is
zero outside the bounds, 1 inside.

Task

Add autograd/gates_basic.v or new gates_clip.v with forward and backward.
Wire into nn/optimizers/ as a clipper option (clip_grad_norm or clip_grad_value).

Files

• autograd/gates_basic.v or new autograd/gates_clip.v
• nn/optimizers/ — add clip option