fix: add canonical encoding check

fuzz/fuzz_targets/fuzz_encoding_roundtrip.rs

@@ -1,8 +1,12 @@
 #![no_main]
 //! Fuzz target: encoding round-trip for all transaction types.
 //!
-//! Invariant: `decode(encode(tx)) == Ok(tx)` and `encode(decode(encode(tx))) == encode(tx)`
-//! for every `PublicTransaction` and `ProgramDeploymentTransaction`.
+//! Invariants exercised:
+//!
+//! 1. **Encode/decode stability** — `encode(decode(encode(tx))) == encode(tx)`.
+//! 2. **Canonical encoding** — if raw fuzzer bytes decode successfully, re-encoding must
+//!    reproduce those exact bytes (`encode(decode(data)) == data`). This catches non-canonical
+//!    encodings that are accepted by the decoder but silently normalised on the way out.
 //!
 //! `PrivacyPreservingTransaction` is excluded because its ZK receipt cannot be
 //! reconstructed in a fuzzing loop.
@@ -42,4 +46,26 @@ fuzz_target!(|data: &[u8]| {
             "INVARIANT VIOLATION: encode(decode(encode(tx))) != encode(tx) for ProgramDeploymentTransaction"
         );
     }
+
+    // ── Test 3: Canonical encoding — raw bytes that decode must re-encode identically ──
+    if let Ok(tx) = PublicTransaction::from_bytes(data) {
+        let re_encoded = tx.to_bytes();
+        assert_eq!(
+            data,
+            re_encoded.as_slice(),
+            "INVARIANT VIOLATION: PublicTransaction decoded from raw fuzzer bytes but \
+             re-encoding differs from the original input (non-canonical encoding accepted)"
+        );
+    }
+
+    // ── Test 4: Canonical encoding for ProgramDeploymentTransaction ──────────────────
+    if let Ok(tx) = ProgramDeploymentTransaction::from_bytes(data) {
+        let re_encoded = tx.to_bytes();
+        assert_eq!(
+            data,
+            re_encoded.as_slice(),
+            "INVARIANT VIOLATION: ProgramDeploymentTransaction decoded from raw fuzzer bytes \
+             but re-encoding differs from the original input (non-canonical encoding accepted)"
+        );
+    }
 });