autograd: tape engine + grad accumulation

Var = Rc<RefCell<VarNode>> on a define-by-run tape: value + optional grad +
parents + backward closure. backward() seeds a scalar loss, walks reverse
topo order, and pushes grads to parents. push_grad always SUMs into the grad
slot — the fan-out accumulation path T3 lacked. Per-crate build.rs emits the
no_cuda cfg (does not propagate); engine gated, grad_check stays host-only.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/xtrain-autodiff/src/lib.rs

@@ -13,3 +13,14 @@
 pub mod finite_diff;
 
 pub use finite_diff::{GradCheckConfig, GradCheckResult, ParamFn, grad_check};
+
+// Tape-based autograd engine + differentiable ops (Phase T4). These call GPU
+// kernels via xtrain-tensor, so they are gated behind `not(no_cuda)` (the
+// per-crate convention); the grad_check harness above stays host-only.
+#[cfg(not(no_cuda))]
+pub mod ops;
+#[cfg(not(no_cuda))]
+pub mod tape;
+
+#[cfg(not(no_cuda))]
+pub use tape::Var;