Include gap in body_margin, fixing mid-phase pruning of in-gap contacts.

The mid-phase BVH descent filter prunes body pairs using body_margin,
which was compiled as the max over geom margins, excluding gap. Broadphase
and the leaf-level test both use margin+gap, so any multi-geom body relying
on gap could silently lose its in-gap contacts when raw AABBs don't
overlap. Single-geom bodies take the leaf-leaf path and were unaffected.

PiperOrigin-RevId: 946967548
Change-Id: I6d92baa296f1a83be68b4dbfd64a96d1c7efd3c4

include/mujoco/mjmodel.h

@@ -378,7 +378,7 @@ typedef struct mjModel_ {
   mjtNum*   body_inertia;         // diagonal inertia in ipos/iquat frame     (nbody x 3)
   mjtNum*   body_invweight0;      // mean inv inert in qpos0 (trn, rot)       (nbody x 2)
   mjtNum*   body_gravcomp;        // antigravity force, units of body weight  (nbody x 1)
-  mjtNum*   body_margin;          // MAX over all geom margins                (nbody x 1)
+  mjtNum*   body_margin;          // MAX over all geom margins+gaps           (nbody x 1)
   mjtNum*   body_user;            // user data                                (nbody x nuser_body)
   int*      body_plugin;          // plugin instance id; -1: not in use       (nbody x 1)
   int*      body_contype;         // OR over all geom contypes                (nbody x 1)