ADR-020 iter-11h-c1 — elementwise exp forward + backward.
Forward: y[i] = exp(x[i]) Backward: dx[i] = dy[i] · y[i] (caller passes y, the forward output, NOT x — autograd-canonical pattern that avoids recompute).
y[i] = exp(x[i])
dx[i] = dy[i] · y[i]