Change mjSpec and mjModel signature mechanism.

The signature now contains the necessary information to safely perform `bind`. The private UIDs are now removed.

Fixes an issue of changing signature when compiling a copy of an mjSpec.

PiperOrigin-RevId: 744670626
Change-Id: Id3c66419cf2afbe78e91bc4b37d2299f5ec00ab1

mjx/mujoco/mjx/_src/support_test.py

@@ -349,7 +349,7 @@ class SupportTest(parameterized.TestCase):
     self.assertEqual(
         str(e.exception),
         'mjSpec signature does not match mjx.Model signature:'
-        ' 5495345807332648606 != 270010677651259353',
+        ' 4300287342280373816 != 9887180086914550999',
     )
 
   _CONTACTS = """