Make constraint island discovery on by default.

Also fix latent bug in mjData serialization.

PiperOrigin-RevId: 799505349
Change-Id: I299e7cc8133fa2ec0b339a7ff3d02543e06778b1

python/mujoco/introspect/enums_test.py

@@ -43,8 +43,7 @@ class EnumsTest(absltest.TestCase):
                                           ('mjENBL_FWDINV', 1<<2),
                                           ('mjENBL_INVDISCRETE', 1<<3),
                                           ('mjENBL_MULTICCD', 1<<4),
-                                          ('mjENBL_ISLAND', 1<<5),
-                                          ('mjNENABLE', 6)))
+                                          ('mjNENABLE', 5)))
 
   # values mostly increment by one with occasional overrides
   def test_mjtGeom(self):  # pylint: disable=invalid-name