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

src/engine/engine_core_util.c

@@ -1077,6 +1077,9 @@ void mj_contactForce(const mjModel* m, const mjData* d, int id, mjtNum result[6]
     } else {
       mju_copy(result, d->efc_force + con->efc_address, con->dim);
     }
+
+    // report the net interface force: the solver's cone force minus the adhesive pull
+    result[0] -= con->adhesion;
   }
 }