Add signature to mjSpec and mjModel and use it to perform safe bind to mjModel and mjData.

PiperOrigin-RevId: 740378879
Change-Id: If14b326942529494f172e7aedcae30195798b458

src/engine/engine_io.c

@@ -2014,6 +2014,9 @@ static void _resetData(const mjModel* m, mjData* d, unsigned char debug_value) {
       }
     }
   }
+
+  // copy signature from model
+  d->signature = m->signature;
 }