feat(ml-dsa): expose c_tilde getter on Signature (#1362)

Add public accessor for the challenge hash component of ML-DSA
signatures, needed by hybrid schemes that share
the challenge across both classical and post-quantum components.

ml-dsa/src/lib.rs

@@ -125,6 +125,11 @@ impl<P: MlDsaParams> Signature<P> {
 
         Some(Self { c_tilde, z, h })
     }
+
+    /// Access the challenge hash `c̃`.
+    pub fn c_tilde(&self) -> &Array<u8, P::Lambda> {
+        &self.c_tilde
+    }
 }
 
 impl<'a, P: MlDsaParams> TryFrom<&'a [u8]> for Signature<P> {