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

PiperOrigin-RevId: 740378879
Change-Id: If14b326942529494f172e7aedcae30195798b458

doc/includes/references.h

@@ -414,6 +414,9 @@ 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;
 typedef enum mjtDisableBit_ {     // disable default feature bitflags
@@ -1451,6 +1454,9 @@ struct mjModel_ {
 
   // paths
   char*     paths;                // paths to assets, 0-terminated            (npaths x 1)
+
+  // compilation signature
+  uint64_t  signature;            // also held by the mjSpec that compiled this model
 };
 typedef struct mjModel_ mjModel;
 struct mjResource_ {
@@ -1713,6 +1719,7 @@ typedef enum mjtOrientation_ {     // type of orientation specifier
 } mjtOrientation;
 typedef struct mjsElement_ {       // element type, do not modify
   mjtObj elemtype;                 // element type
+  uint64_t signature;              // compilation signature
 } mjsElement;
 typedef struct mjsCompiler_ {      // compiler options
   mjtByte autolimits;              // infer "limited" attribute based on range