ReLU network algebra and approximation

This entrypoint collects the ReLU-specific proof layer. It includes:

• exact algebraic bridges between TorchLean's two-layer MLP spec and affine/ReLU expressions;
• multiplication approximation on boxes by ReLU networks; and
• compact-set approximation by coordinate-polynomial density plus ReLU realizations.

This module stays separate from the architecture files: the model files define how an MLP computes, while this chapter proves what ReLU networks can represent and approximate.