Add boolean flags for gravity compensation and surface velocity in mjModel.

These fields (`flg_gravcomp` and `flg_surfacevel`) replace the fast-path checks originally guarded by `ngravcomp` and (recently) `nsurfacevel`. Since the engine uses these integers only as flags (zero vs non-zero), migrating them to actual booleans makes them writeable from the Python bindings at runtime without violating size/dimension constraints.

The legacy integer field `ngravcomp` is marked as deprecated and will be removed in a future release.

PiperOrigin-RevId: 949779204
Change-Id: Ifab1f026063a4239302e6ad689663b611b59dda8

python/mujoco/bindings_test.py

@@ -169,6 +169,25 @@ class MuJoCoBindingsTest(parameterized.TestCase):
         self.data.qpos, [0.12345] * len(self.data.qpos)
     )
 
+  def test_flg_gravcomp_and_surfacevel_properties(self):
+    self.assertFalse(self.model.flg_gravcomp)
+    self.assertFalse(self.model.flg_surfacevel)
+    self.assertEqual(self.model.ngravcomp, 0)
+
+    self.model.flg_gravcomp = True
+    self.assertTrue(self.model.flg_gravcomp)
+    self.assertEqual(self.model.ngravcomp, 1)
+
+    self.model.flg_surfacevel = True
+    self.assertTrue(self.model.flg_surfacevel)
+
+    self.model.flg_gravcomp = False
+    self.assertFalse(self.model.flg_gravcomp)
+    self.assertEqual(self.model.ngravcomp, 0)
+
+    self.model.flg_surfacevel = False
+    self.assertFalse(self.model.flg_surfacevel)
+
   def test_array_is_a_view(self):
     qpos_ref = self.data.qpos
     self.data.qpos = 0.789

python/mujoco/introspect/structs.py

@@ -1368,11 +1368,6 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  type=ValueType(name='mjtSize'),
                  doc='number of bodies with nonzero gravcomp',
              ),
-             StructFieldDecl(
-                 name='nsurfacevel',
-                 type=ValueType(name='mjtSize'),
-                 doc='number of geoms with nonzero surfacevel',
-             ),
              StructFieldDecl(
                  name='nemax',
                  type=ValueType(name='mjtSize'),
@@ -1428,6 +1423,16 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  type=ValueType(name='mjtSize'),
                  doc='number of bytes in buffer',
              ),
+             StructFieldDecl(
+                 name='flg_gravcomp',
+                 type=ValueType(name='mjtBool'),
+                 doc='whether any body has nonzero gravcomp',
+             ),
+             StructFieldDecl(
+                 name='flg_surfacevel',
+                 type=ValueType(name='mjtBool'),
+                 doc='whether any geom has nonzero surfacevel',
+             ),
              StructFieldDecl(
                  name='opt',
                  type=ValueType(name='mjOption'),

python/mujoco/structs.cc

@@ -350,6 +350,21 @@ This is useful for example when the MJB is not available as a file on disk.)"));
   mjModel.def_readonly("vis", &MjModelWrapper::vis);
   mjModel.def_readonly("stat", &MjModelWrapper::stat);
 
+  mjModel.def_property(
+      "flg_gravcomp",
+      [](const MjModelWrapper& m) { return m.get()->flg_gravcomp; },
+      [](MjModelWrapper& m, bool val) {
+        m.get()->flg_gravcomp = val;
+        m.get()->ngravcomp = val ? 1 : 0;
+      });
+
+  mjModel.def_property(
+      "flg_surfacevel",
+      [](const MjModelWrapper& m) { return m.get()->flg_surfacevel; },
+      [](MjModelWrapper& m, bool val) {
+        m.get()->flg_surfacevel = val;
+      });
+
 #define X(var)                   \
   mjModel.def_property_readonly( \
       #var, [](const MjModelWrapper& m) { return m.get()->var; });