Fix the PolynomialDegree module

In Bolton's library, the PolynomialDegree module is used for the Baby SNARK soundness proof. But it seems we can't port those lemmas directly.

One needs to analyse what makes lemmas broken in Lean4 and, perhaps, rewrite proofs and formulations.

Start by locating the PolynomialDegree module and comparing its Baby SNARK soundness lemmas with the corresponding material in Bolton's library. Identify which lemmas or formulations fail in Lean4 and determine the required proof or statement changes. Done means the module's lemmas are ported and the project checks successfully.

Written by the indexing model from the issue text.