Skip Connections With Inertia, in JAX/Flax NNX

A runnable companion: the residual block as a forward-Euler step, then the momentum residual network as a Flax NNX module with one extra state, a velocity the blocks write into. Train both on the rings task a first-order flow cannot separate exactly, watch the training crystallize, and run the trained network exactly backward until floating point, amplified by 1/mu per layer, steals the past.