Formalise Case 2 in the Baby SNARK soundness proof

The soundness theorem has two subcases depending on whether the constraints hold, see P. 6 by Miller, Zhang and Kanjalkar. Bolton formalised only the first case where all constraints hold everywhere.

The issue is formalise case 2 where some of constraints don't have to be valid somewhere.

Start by locating the soundness theorem and reviewing the cited page 6 from Miller, Zhang and Kanjalkar. Formalise the second subcase, where some constraints may be invalid at some points, and confirm that the completed soundness proof checks.

Written by the indexing model from the issue text.