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

include/mujoco/mjmodel.h

@@ -332,7 +332,6 @@ typedef struct mjModel_ {
   mjtSize nnames_map;             // number of slots in the names hash map
   mjtSize nJmom;                  // number of non-zeros in sparse actuator_moment matrix
   mjtSize ngravcomp;              // number of bodies with nonzero gravcomp
-  mjtSize nsurfacevel;            // number of geoms with nonzero surfacevel
   mjtSize nemax;                  // number of potential equality-constraint rows
   mjtSize njmax;                  // number of available rows in constraint Jacobian (legacy)
   mjtSize nconmax;                // number of potential contacts in contact list (legacy)
@@ -347,6 +346,11 @@ typedef struct mjModel_ {
   mjtSize narena;                 // number of bytes in the mjData arena (inclusive of stack)
   mjtSize nbuffer;                // number of bytes in buffer
 
+  // ------------------------------- flags
+
+  mjtBool flg_gravcomp;           // whether any body has nonzero gravcomp
+  mjtBool flg_surfacevel;         // whether any geom has nonzero surfacevel
+
   // ------------------------------- options and statistics
 
   mjOption opt;                   // physics options

include/mujoco/mjxmacro.h

@@ -247,7 +247,6 @@
     X( nnames_map )         \
     X( nJmom )              \
     X( ngravcomp )          \
-    X( nsurfacevel )        \
     X( nemax )              \
     X( njmax )              \
     X( nconmax )            \