Return false instead of panicking on zero GKR root denominator

Author: nicole-graus

crypto/stark/src/gkr.rs

@@ -2047,6 +2047,14 @@ pub fn gkr_verify_batch<E: IsField>(
                 // instance i's child layer has (n_layers[i] - n_remaining + 1) variables,
                 // so the parent has (n_layers[i] - n_remaining) variables.
                 let parent_num_vars_i = n_layers_by_instance[i] - n_remaining;
+                if num_rounds < parent_num_vars_i {
+                    return Err(GkrError::InvalidTree {
+                        reason: format!(
+                            "layer {}: num_rounds ({}) < parent_num_vars for instance {} ({})",
+                            layer_idx, num_rounds, i, parent_num_vars_i,
+                        ),
+                    });
+                }
                 let sumcheck_n_unused = num_rounds - parent_num_vars_i;
 
                 // eq evaluation: the prover builds eq from the instance-specific eval point

crypto/stark/src/verifier.rs

@@ -990,7 +990,14 @@ pub trait IsStarkVerifier<
                     let contrib = if *root_d == FieldElement::one() {
                         root_n.clone()
                     } else {
-                        root_n * &root_d.inv().expect("GKR root denominator must be non-zero")
+                        match root_d.inv() {
+                            Ok(inv) => root_n * &inv,
+                            Err(_) => {
+                                #[cfg(not(feature = "test_fiat_shamir"))]
+                                error!("GKR root denominator is zero — invalid proof");
+                                return false;
+                            }
+                        }
                     };
                     total = total + &contrib;
                 }