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/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; });