Sumcheck Speedup Involving PowPoly

[Sun-Jc]

• Avoided the creation of the full power of tau before sumcheck
• Avoided binding the power of tau polynomial during sumcheck
• Applied the technique adapted from Speeding Up Sum-Check Proving (see also Optimized Sumcheck Involving EQ Polynomials Nova#409)

[Copilot]

Pull Request Overview

This PR refactors the sum-check protocol to optimize the handling of the power-of-tau polynomial by splitting it into an outer product representation of two smaller polynomials. This reduces memory usage and improves performance by avoiding the explicit computation of the full tensor product.

Key changes:

• Splits the power-of-tau polynomial into left and right components for efficient outer product computation
• Adds a new optimized evaluation function that computes cubic polynomial evaluation points using the split representation
• Converts split_evals from an instance method to a static method to avoid unnecessary object construction

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/sumcheck.rs	Fixes bug in length calculation (line 362), adds new compute_eval_points_cubic_with_additive_term_with_outer_pow function for optimized evaluation with split tau polynomials, and updates prove_cubic_with_additive_term_batched_zk to use split polynomial representation
src/polys/power.rs	Refactors split_evals from instance method to static method, removing dependency on self
src/neutronnova_zk.rs	Updates tau polynomial construction to use split representation and passes split components to sum-check proving function

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