Optional exact reconstruction checker

The following standard-library Python listing reconstructs the raw contact matrix from the stated sparse tensors. It checks the blind row, species complementation, the original pivots and minor, the three-row and four-code fits, and the stated finite bounds.

Its target calculation uses the proved restricted cubic formula. It is not an independent Einstein-vertex implementation. The listing has not been executed in this interface and is not a premise of the proofs.

Slots are zero-based in the code. Matchings are generated by pairing the smallest remaining slot with every possible partner and recursing. This gives each matching exactly once.