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

mjx/mujoco/mjx/_src/forward.py

@@ -228,7 +228,7 @@ def fwd_actuation(m: Model, d: Data) -> Data:
 
   qfrc_actuator = d._impl.actuator_moment.T @ force
 
-  if m.ngravcomp:
+  if m.flg_gravcomp:
     # actuator-level gravity compensation, skip if added as passive force
     qfrc_actuator += d.qfrc_gravcomp * m.jnt_actgravcomp[m.dof_jntid]
 

mjx/mujoco/mjx/_src/passive.py

@@ -145,7 +145,7 @@ def passive(m: Model, d: Data) -> Data:
   qfrc_passive = _spring_damper(m, d)
   qfrc_gravcomp = jp.zeros(m.nv)
 
-  if m.ngravcomp and not m.opt.disableflags & DisableBit.GRAVITY:
+  if m.flg_gravcomp and not m.opt.disableflags & DisableBit.GRAVITY:
     qfrc_gravcomp = _gravcomp(m, d)
     # add gravcomp unless added via actuators
     qfrc_passive += qfrc_gravcomp * (1 - m.jnt_actgravcomp[m.dof_jntid])

mjx/mujoco/mjx/_src/types.py

@@ -641,6 +641,8 @@ class Model(PyTreeNode):
   nJmom: int  # pylint:disable=invalid-name
   nJten: int  # pylint:disable=invalid-name
   ngravcomp: int
+  flg_gravcomp: bool
+  flg_surfacevel: bool
   nuserdata: int
   nsensordata: int
   npluginstate: int