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

PiperOrigin-RevId: 740378879
Change-Id: If14b326942529494f172e7aedcae30195798b458

python/mujoco/introspect/structs.py

@@ -4454,6 +4454,11 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  doc='paths to assets, 0-terminated',
                  array_extent=('npaths',),
              ),
+             StructFieldDecl(
+                 name='signature',
+                 type=ValueType(name='uint64_t'),
+                 doc='also held by the mjSpec that compiled this model',
+             ),
          ),
      )),
     ('mjThreadPool',
@@ -6037,6 +6042,11 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  type=ValueType(name='uintptr_t'),
                  doc='thread pool pointer',
              ),
+             StructFieldDecl(
+                 name='signature',
+                 type=ValueType(name='uint64_t'),
+                 doc='also held by the mjSpec that compiled the model',
+             ),
          ),
      )),
     ('mjvPerturb',
@@ -9044,6 +9054,11 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  type=ValueType(name='mjtObj'),
                  doc='element type',
              ),
+             StructFieldDecl(
+                 name='signature',
+                 type=ValueType(name='uint64_t'),
+                 doc='compilation signature',
+             ),
          ),
      )),
     ('mjsCompiler',