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

PiperOrigin-RevId: 740378879
Change-Id: If14b326942529494f172e7aedcae30195798b458

include/mujoco/mjdata.h

@@ -442,6 +442,9 @@ struct mjData_ {
 
   // thread pool pointer
   uintptr_t threadpool;
+
+  // compilation signature
+  uint64_t  signature;       // also held by the mjSpec that compiled the model
 };
 typedef struct mjData_ mjData;