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/mjmodel.h

@@ -350,6 +350,7 @@ typedef struct mjModel_ {
 
   mjtBool flg_gravcomp;           // whether any body has nonzero gravcomp
   mjtBool flg_surfacevel;         // whether any geom has nonzero surfacevel
+  mjtBool flg_adhesion;           // whether any geom or pair has nonzero adhesion
 
   // ------------------------------- options and statistics
 
@@ -478,6 +479,7 @@ typedef struct mjModel_ {
   mjtNum*   geom_margin;          // geometric inflation for contact          (ngeom x 1)
   mjtNum*   geom_gap;             // additional contact detection buffer      (ngeom x 1)
   mjtNum*   geom_surfacevel;      // surface velocity in local frame: lin,ang (ngeom x 6)
+  mjtNum*   geom_adhesion;        // adhesive force of contacts               (ngeom x 1)
   mjtNum*   geom_fluid;           // fluid interaction parameters             (ngeom x mjNFLUID)
   mjtNum*   geom_user;            // user data                                (ngeom x nuser_geom)
   float*    geom_rgba;            // rgba when material is omitted            (ngeom x 4)
@@ -721,6 +723,7 @@ typedef struct mjModel_ {
   mjtNum*   pair_solimp;          // solver impedance: contact                (npair x mjNIMP)
   mjtNum*   pair_margin;          // geometric inflation for contact          (npair x 1)
   mjtNum*   pair_gap;             // additional contact detection buffer      (npair x 1)
+  mjtNum*   pair_adhesion;        // adhesive force of contacts               (npair x 1)
   mjtNum*   pair_friction;        // tangent1, 2, spin, roll1, 2              (npair x 5)
 
   // excluded body pairs for collision detection