fix(bb): allocate gemini_masking_poly at dyadic virtual size

Author: AztecBot

barretenberg/cpp/src/barretenberg/ultra_honk/oink_prover.cpp

@@ -186,9 +186,12 @@ template <typename Flavor> void OinkProver<Flavor>::commit_to_z_perm()
 template <typename Flavor> void OinkProver<Flavor>::commit_to_masking_poly()
 {
     if constexpr (flavor_has_gemini_masking<Flavor>()) {
-        // Gemini masking poly only needs to cover the actual polynomial extent, not full dyadic size
+        // Gemini masking poly only needs random content up to the actual polynomial extent, but its
+        // virtual_size must match the rest of the sumcheck entities so the sumcheck main loop can
+        // safely read the zero-padded virtual region past end_index.
         const size_t polynomial_size = prover_instance->polynomials.max_end_index();
-        prover_instance->polynomials.gemini_masking_poly = Polynomial<FF>::random(polynomial_size);
+        prover_instance->polynomials.gemini_masking_poly =
+            Polynomial<FF>::random(polynomial_size, prover_instance->dyadic_size(), /*start_index=*/0);
 
         // Commit to the masking polynomial and send to transcript
         auto masking_commitment = commitment_key.commit(prover_instance->polynomials.gemini_masking_poly);