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/XMLschema.rst

@@ -911,6 +911,9 @@
             .. grid-item::
                :ref:`surfacevel<body-geom-surfacevel>`
 
+            .. grid-item::
+               :ref:`adhesion<body-geom-adhesion>`
+
             .. grid-item::
                :ref:`fromto<body-geom-fromto>`
 
@@ -1379,6 +1382,9 @@
                .. grid-item::
                   :ref:`surfacevel<composite-geom-surfacevel>`
 
+               .. grid-item::
+                  :ref:`adhesion<composite-geom-adhesion>`
+
 
          .. dropdown:: :ref:`site<composite-site>` :octicon:`dot`
 
@@ -1871,6 +1877,9 @@
             .. grid-item::
                :ref:`margin<contact-pair-margin>`
 
+            .. grid-item::
+               :ref:`adhesion<contact-pair-adhesion>`
+
 
       .. dropdown:: :ref:`exclude<contact-exclude>` |*|
 
@@ -5504,6 +5513,9 @@
             .. grid-item::
                :ref:`surfacevel<default-geom-surfacevel>`
 
+            .. grid-item::
+               :ref:`adhesion<default-geom-adhesion>`
+
             .. grid-item::
                :ref:`fromto<default-geom-fromto>`
 
@@ -5726,6 +5738,9 @@
             .. grid-item::
                :ref:`margin<default-pair-margin>`
 
+            .. grid-item::
+               :ref:`adhesion<default-pair-adhesion>`
+
 
       .. dropdown:: :ref:`equality<default-equality>` :octicon:`dot`