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

src/xml/xml_native_writer.cc

@@ -498,6 +498,7 @@ void mjXWriter::OneGeom(XMLElement* elem, const mjCGeom* geom, mjCDef* def, stri
   WriteAttr(elem, "margin", 1, &geom->margin, &def->Geom().margin);
   WriteAttr(elem, "gap", 1, &geom->gap, &def->Geom().gap);
   WriteAttr(elem, "surfacevel", 6, geom->surfacevel, def->Geom().surfacevel, true);
+  WriteAttr(elem, "adhesion", 1, &geom->adhesion, &def->Geom().adhesion);
   WriteAttrKey(elem, "fluidshape",
                fluid_map, 2, geom->fluid_ellipsoid, def->Geom().fluid_ellipsoid);
   WriteAttr(elem, "fluidcoef", 5, geom->fluid_coefs, def->Geom().fluid_coefs);
@@ -672,6 +673,7 @@ void mjXWriter::OnePair(XMLElement* elem, const mjCPair* pair, mjCDef* def) {
   WriteAttrInt(elem, "condim", pair->condim, def->Pair().spec.condim);
   WriteAttr(elem, "margin", 1, &pair->margin, &def->Pair().spec.margin);
   WriteAttr(elem, "gap", 1, &pair->gap, &def->Pair().spec.gap);
+  WriteAttr(elem, "adhesion", 1, &pair->adhesion, &def->Pair().spec.adhesion);
   WriteAttr(elem, "solref", mjNREF, pair->solref, def->Pair().spec.solref, true);
   WriteAttr(elem, "solreffriction", mjNREF, pair->solreffriction, def->Pair().spec.solreffriction,
             true);