BackwardOps

NF reverse-mode runtime-to-spec approximation lemmas.

The public theorem is NFBackend.backprop_approx: a reverse graph built from sound local NF nodes has a backpropagated rounded context within explicit per-entry error bounds of the supplied spec reverse pass. This is approximation, not a derivative-correctness proof. The local node families cover sparse VJP contexts, context accumulation, primitive operations, and linear algebra.