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

python/mujoco/structs.cc

@@ -365,6 +365,13 @@ This is useful for example when the MJB is not available as a file on disk.)"));
         m.get()->flg_surfacevel = val;
       });
 
+  mjModel.def_property(
+      "flg_adhesion",
+      [](const MjModelWrapper& m) { return m.get()->flg_adhesion; },
+      [](MjModelWrapper& m, bool val) {
+        m.get()->flg_adhesion = val;
+      });
+
 #define X(var)                   \
   mjModel.def_property_readonly( \
       #var, [](const MjModelWrapper& m) { return m.get()->var; });
@@ -770,6 +777,7 @@ This is useful for example when the MJB is not available as a file on disk.)"));
       })
   X(dist);
   X(includemargin);
+  X(adhesion);
   X(mu);
   X(dim);
   X(geom1);
@@ -821,6 +829,7 @@ This is useful for example when the MJB is not available as a file on disk.)"));
   XN(mjtNum, solref);
   XN(mjtNum, solreffriction);
   XN(mjtNum, solimp);
+  X(mjtNum, adhesion);
   X(mjtNum, mu);
   XN(mjtNum, H);
   X(int, dim);