Add sum-over-hypercube invariant test for eq polynomial

[Bisht13]

Context

PR #317 added unit tests for the sumcheck eq-polynomial utilities (calculate_eq, eval_eq, calculate_evaluations_over_boolean_hypercube_for_eq, transpose/multiply helpers). Coverage is solid, but one fundamental mathematical invariant is untested.

Missing test

For any r ∈ F^n, the sum of eq(r, x) over all x ∈ {0,1}^n must equal 1:

$$\sum_{x \in {0,1}^n} \text{eq}(r, x) = 1$$

This is a core property of the multilinear extension of the equality polynomial.

Why it matters

The existing test_eq_hypercube_len4 cross-validates calculate_evaluations_over_boolean_hypercube_for_eq against calculate_eq entry-by-entry — but if calculate_eq itself has a bug, both sides agree on the wrong answer. The sum property is a self-checking invariant that doesn't depend on a second implementation being correct.

Suggested test

#[test]
fn test_eq_hypercube_sums_to_one() {
    let r = [fe(2), fe(3), fe(4), fe(5)];
    let evals = calculate_evaluations_over_boolean_hypercube_for_eq(&r, 16);
    let sum: FieldElement = evals.iter().copied().sum();
    assert_eq!(sum, fe(1));
}

Location

provekit/common/src/utils/sumcheck.rs, in the #[cfg(test)] mod tests block alongside the existing hypercube tests.