Add geom adhesion: contacts that pull, via translated friction cones.

https://youtu.be/GioWwB36XHI

The new geom attribute adhesion (units of force, signed; pair-level
override) translates the contact friction cone along its normal so
that the force origin lies strictly inside it. Consequences: each
contact can pull with up to the given force before breaking, and the
tangential friction budget becomes mu*(f_N + adhesion) -- the
Mohr-Coulomb yield condition with cohesion c = mu*adhesion -- so
lightly-squeezed grasps retain a guaranteed friction floor.

A translated cone factors exactly into {constant attractive force}
+ {original cone}, so no solver kernels change. The implementation is
this factorization: a constant attraction along contact normals
accumulated into the new mjData.qfrc_adhesion (summed into
qfrc_passive), plus a bias of adhesive contact rows' reference
acceleration (aref += R*adhesion), which makes resting penetration
exactly independent of adhesion. Contacts of adhesive pairs remain
active throughout the gap zone, producing rows with positive violation
whose reference acceleration pulls: a tether that resists pull-off
smoothly, captures objects released within the band into steady
contact, and detaches at the specified force. Adhesion values of the
two geoms combine by sum; explicit pairs override.

mj_contactForce reports the net interface force (cone force minus the
adhesive pull), whose normal component can now be negative. Negative
adhesion is allowed and produces a repulsive offset (air hockey).

PiperOrigin-RevId: 950858148
Change-Id: I879c08eba7ae501e5c0f8c2f807167344da4c2bc

include/mujoco/mjxmacro.h

@@ -386,6 +386,7 @@
     X   ( mjtNum,  geom_margin,           ngeom,         1                    ) \
     X   ( mjtNum,  geom_gap,              ngeom,         1                    ) \
     X   ( mjtNum,  geom_surfacevel,       ngeom,         6                    ) \
+    X   ( mjtNum,  geom_adhesion,         ngeom,         1                    ) \
     XNV ( mjtNum,  geom_fluid,            ngeom,         mjNFLUID             ) \
     X   ( mjtNum,  geom_user,             ngeom,         MJ_M(nuser_geom)     ) \
     X   ( float,   geom_rgba,             ngeom,         4                    )
@@ -627,6 +628,7 @@
     X   ( mjtNum,  pair_solimp,           npair,         mjNIMP               ) \
     X   ( mjtNum,  pair_margin,           npair,         1                    ) \
     X   ( mjtNum,  pair_gap,              npair,         1                    ) \
+    X   ( mjtNum,  pair_adhesion,         npair,         1                    ) \
     X   ( mjtNum,  pair_friction,         npair,         5                    )
 
 #define MJMODEL_POINTERS_EXCLUDE                                                \
@@ -911,6 +913,7 @@
     X   ( mjtNum,    qfrc_damper,       nv,          1           ) \
     X   ( mjtNum,    qfrc_gravcomp,     nv,          1           ) \
     X   ( mjtNum,    qfrc_fluid,        nv,          1           ) \
+    X   ( mjtNum,    qfrc_adhesion,     nv,          1           ) \
     X   ( mjtNum,    qfrc_passive,      nv,          1           ) \
     X   ( mjtNum,    subtree_linvel,    nbody,       3           ) \
     X   ( mjtNum,    subtree_angmom,    nbody,       3           ) \