fix misleading comments: Mz uses c (not r) in PPSNARK

[GarmashAlex]

Replace r with c in comments describing Mz and its evaluation in ppsnark.rs. The code already uses the Fiat–Shamir challenge c to form Mz = Az + cBz + c^2Cz and its evaluation at tau; only the comments were incorrect.

[Copilot]

Pull Request Overview

This PR fixes misleading comments in the ppsnark.rs file that incorrectly referenced the variable 'r' when describing the Mz polynomial and its evaluation. The code correctly uses the Fiat-Shamir challenge 'c', but the comments were inconsistent with the implementation.

• Corrected variable references in comments from 'r' to 'c' for Mz polynomial construction
• Updated evaluation comments to match the actual code that uses challenge 'c'

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