test(cubecrypt): add persistent XTS roundtrip + non-AEAD test (was only ad-hoc verified)

XTS was previously covered only by a deleted throwaway example. Fold the
verification into the committed suite so it runs under ./check: 16-multiple
roundtrip, short-plaintext pad/strip roundtrip, multi-block sector, and the
correct non-AEAD wrong-key property (wrong key opens to different garbage).

cubecrypt/src/lib.rs

@@ -120,6 +120,29 @@ mod tests {
         }
     }
 
+    #[test]
+    fn xts_roundtrip_and_non_authenticated() {
+        // XTS needs 16-byte-multiple plaintext for the raw mode; seal pads
+        // non-multiples and open strips them, so both shapes round-trip.
+        let key = transform::derive_key(b"xts-root-key-material-32b", b"salt");
+        // exact 16-multiple (3 blocks)
+        let aligned: Vec<u8> = (0u8..48).collect();
+        let env = transform::seal(TransformId::Aes256Xts, &key, &aligned);
+        assert_eq!(transform::open(&key, &env).unwrap(), aligned);
+        // short plaintext (padded/ stripped)
+        let short = b"hello";
+        let env_s = transform::seal(TransformId::Aes256Xts, &key, short);
+        assert_eq!(transform::open(&key, &env_s).unwrap(), short);
+        // multi-block sector
+        let big = vec![0xABu8; 64];
+        let env_b = transform::seal(TransformId::Aes256Xts, &key, &big);
+        assert_eq!(transform::open(&key, &env_b).unwrap(), big);
+        // XTS is non-AEAD: wrong key opens silently to *different* garbage.
+        let wrong = transform::derive_key(b"xts-root-key-material-32b", b"other");
+        let got = transform::open(&wrong, &env).unwrap();
+        assert_ne!(got, aligned, "XTS wrong-key must NOT reproduce plaintext");
+    }
+
     #[test]
     fn wrong_key_fails_auth() {
         let key = transform::derive_key(b"key-A-material", b"salt");