refactor(stark): the grinding factor is not "security_bits" (#979)

Three local bindings read `proof_options.grinding_factor` and named it
`security_bits`, then passed it to functions whose own parameter is
`grinding_factor`:

    prover.rs:2286    let security_bits = air.context().proof_options.grinding_factor;
    verifier.rs:1582  let security_bits = air.context().proof_options.grinding_factor;
    verifier.rs:1665  let security_bits = air.context().proof_options.grinding_factor;

Grinding is not the security level. It is one term in the query round's error,
and it cannot move the commit-phase term at all — so a reader who takes these
bindings at their word concludes the proof carries 20 bits of security, or that
raising the grinding factor raises security generally. Neither follows.

`grinding_factor` is what every function behind these call sites already calls
the argument (`grinding::is_valid_nonce`, `generate_nonce`,
`generate_nonce_maybe_gpu`), so the rename removes a mismatch rather than
introducing a new convention. Nothing is shadowed: the name appeared in these
two files only as the struct field being read.

Eight identifier occurrences, no behaviour, no field moved. The one operator-
facing message in the area already says the right thing — `error!("Grinding
factor not satisfied")` — so no text needed correcting alongside the names.

Author: MauroToscano

crypto/stark/src/prover.rs

@@ -2283,11 +2283,11 @@ pub trait IsStarkProver<
         // grinding: generate nonce and append it to the transcript
         #[cfg(feature = "instruments")]
         let t_sub = Instant::now();
-        let security_bits = air.context().proof_options.grinding_factor;
+        let grinding_factor = air.context().proof_options.grinding_factor;
         let mut nonce = None;
-        if security_bits > 0 {
+        if grinding_factor > 0 {
             let nonce_value =
-                grinding::generate_nonce_maybe_gpu(&transcript.state(), security_bits)
+                grinding::generate_nonce_maybe_gpu(&transcript.state(), grinding_factor)
                     .expect("nonce not found");
             transcript.append_bytes(&nonce_value.to_be_bytes());
             nonce = Some(nonce_value);

crypto/stark/src/verifier.rs

@@ -1579,9 +1579,9 @@ pub trait IsStarkVerifier<
         }
 
         // Receive grinding value
-        let security_bits = air.context().proof_options.grinding_factor;
+        let grinding_factor = air.context().proof_options.grinding_factor;
         let mut grinding_seed = [0u8; 32];
-        if security_bits > 0
+        if grinding_factor > 0
             && let Some(nonce_value) = proof.nonce()
         {
             grinding_seed = transcript.state();
@@ -1662,10 +1662,10 @@ pub trait IsStarkVerifier<
         );
 
         // verify grinding
-        let security_bits = air.context().proof_options.grinding_factor;
-        if security_bits > 0 {
+        let grinding_factor = air.context().proof_options.grinding_factor;
+        if grinding_factor > 0 {
             let nonce_is_valid = proof.nonce().is_some_and(|nonce_value| {
-                grinding::is_valid_nonce(&challenges.grinding_seed, nonce_value, security_bits)
+                grinding::is_valid_nonce(&challenges.grinding_seed, nonce_value, grinding_factor)
             });
 
             if !nonce_is_valid {