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/introspect/structs.py

@@ -1433,6 +1433,11 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  type=ValueType(name='mjtBool'),
                  doc='whether any geom has nonzero surfacevel',
              ),
+             StructFieldDecl(
+                 name='flg_adhesion',
+                 type=ValueType(name='mjtBool'),
+                 doc='whether any geom or pair has nonzero adhesion',
+             ),
              StructFieldDecl(
                  name='opt',
                  type=ValueType(name='mjOption'),
@@ -2263,6 +2268,14 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  doc='surface velocity in local frame: lin,ang',
                  array_extent=('ngeom', 6),
              ),
+             StructFieldDecl(
+                 name='geom_adhesion',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjtNum'),
+                 ),
+                 doc='adhesive force of contacts',
+                 array_extent=('ngeom',),
+             ),
              StructFieldDecl(
                  name='geom_fluid',
                  type=PointerType(
@@ -4031,6 +4044,14 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  doc='additional contact detection buffer',
                  array_extent=('npair',),
              ),
+             StructFieldDecl(
+                 name='pair_adhesion',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjtNum'),
+                 ),
+                 doc='adhesive force of contacts',
+                 array_extent=('npair',),
+             ),
              StructFieldDecl(
                  name='pair_friction',
                  type=PointerType(
@@ -5409,6 +5430,11 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  ),
                  doc='constraint solver impedance',
              ),
+             StructFieldDecl(
+                 name='adhesion',
+                 type=ValueType(name='mjtNum'),
+                 doc='adhesive force along the contact normal',
+             ),
              StructFieldDecl(
                  name='mu',
                  type=ValueType(name='mjtNum'),
@@ -6415,6 +6441,14 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  doc='passive fluid force',
                  array_extent=('nv',),
              ),
+             StructFieldDecl(
+                 name='qfrc_adhesion',
+                 type=PointerType(
+                     inner_type=ValueType(name='mjtNum'),
+                 ),
+                 doc='passive contact adhesion force',
+                 array_extent=('nv',),
+             ),
              StructFieldDecl(
                  name='qfrc_passive',
                  type=PointerType(
@@ -7983,6 +8017,11 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  ),
                  doc='surface velocity in local frame: linear, angular',
              ),
+             StructFieldDecl(
+                 name='adhesion',
+                 type=ValueType(name='double'),
+                 doc='adhesive force of contacts',
+             ),
              StructFieldDecl(
                  name='mass',
                  type=ValueType(name='double'),
@@ -9256,6 +9295,11 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  type=ValueType(name='double'),
                  doc='additional contact detection buffer',
              ),
+             StructFieldDecl(
+                 name='adhesion',
+                 type=ValueType(name='double'),
+                 doc='adhesive force of contacts',
+             ),
              StructFieldDecl(
                  name='friction',
                  type=ArrayType(