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

doc/changelog.rst

@@ -21,6 +21,16 @@ Engine
   velocity relative to the moving surface rather than to the geom, since that is the quantity the constraint acts
   on; for geoms without :at:`surfacevel` the two are identical. Contact-point visualization draws an arrow along the
   surface velocity at contacts with moving surfaces.
+
+.. youtube:: GioWwB36XHI
+   :align: right
+   :width: 35%
+
+- Added :ref:`geom/adhesion<body-geom-adhesion>` and :ref:`pair/adhesion<contact-pair-adhesion>`: an adhesive force
+  associated with a contact, useful for modeling sticky materials. Contacts can pull with up to the given force before
+  breaking, and the friction budget becomes :math:`\mu(f_N + \text{adhesion})`. Combined with :ref:`gap<body-geom-gap>`,
+  adhesive contacts apply "adhesion at a distance", useful for modeling magnets. Resting penetration is unaffected by
+  adhesion. :ref:`mj_contactForce` reports the net interface force, whose normal component can now be negative.
 - Replaced midpoint integration of free bodies with :ref:`gyroscopic derivatives<geFreeBody>` in the ``implicitfast``
   :ref:`integrator<geIntegrators>`: the bias-force derivative of every standalone free body is applied via a local
   unsymmetric solve of its decoupled block, making ``implicitfast`` identical to ``implicit`` for such bodies.
@@ -56,7 +66,8 @@ Engine
 .. admonition:: Breaking ABI changes
    :class: caution
 
-   - Added ``texid``, ``texuniform`` and ``texrepeat`` fields to ``mjvGeom``.
+   - Added ``texid``, ``texuniform`` and ``texrepeat`` fields to :ref:`mjvGeom`.
+   - The :ref:`mjContact`` struct gained an ``adhesion`` member, changing its size and layout.
 
 .. admonition:: Bug fixes
    :class: admonition