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/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'),