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_reader.cc

@@ -238,7 +238,8 @@ std::vector<const char*> MJCF[nMJCF] = {
         {"geom", "?", "type", "pos", "quat", "contype", "conaffinity", "condim",
             "group", "priority", "size", "material", "friction", "mass", "density",
             "shellinertia", "solmix", "solref", "solimp",
-            "margin", "gap", "surfacevel", "fromto", "axisangle", "xyaxes", "zaxis", "euler",
+            "margin", "gap", "surfacevel", "adhesion", "fromto", "axisangle", "xyaxes", "zaxis",
+            "euler",
             "hfield", "mesh", "fitscale", "rgba", "fluidshape", "fluidcoef", "user"},
         {"site", "?", "type", "group", "pos", "quat", "material",
             "size", "fromto", "axisangle", "xyaxes", "zaxis", "euler", "rgba", "user"},
@@ -249,7 +250,7 @@ std::vector<const char*> MJCF[nMJCF] = {
             "directional", "type", "castshadow", "active", "attenuation", "cutoff", "exponent",
             "ambient", "diffuse", "specular", "mode"},
         {"pair", "?", "condim", "friction", "solref", "solreffriction", "solimp",
-         "gap", "margin"},
+         "gap", "margin", "adhesion"},
         {"equality", "?", "active", "solref", "solimp"},
         {"tendon", "?", "group", "limited", "range",
             "solreflimit", "solimplimit", "solreffriction", "solimpfriction",
@@ -349,9 +350,9 @@ std::vector<const char*> MJCF[nMJCF] = {
         {"geom", "*", "name", "class", "type", "contype", "conaffinity", "condim",
             "group", "priority", "size", "material", "friction", "mass", "density",
             "shellinertia", "solmix", "solref", "solimp",
-            "margin", "gap", "surfacevel", "fromto", "pos", "quat", "axisangle", "xyaxes",
-            "zaxis", "euler", "hfield", "mesh", "fitscale", "rgba", "fluidshape", "fluidcoef",
-            "user"},
+            "margin", "gap", "surfacevel", "adhesion", "fromto", "pos", "quat", "axisangle",
+            "xyaxes", "zaxis", "euler", "hfield", "mesh", "fitscale", "rgba", "fluidshape",
+            "fluidcoef", "user"},
         {"<"},
             {"plugin", "*", "plugin", "instance"},
             {"<"},
@@ -381,7 +382,8 @@ std::vector<const char*> MJCF[nMJCF] = {
             {"skin", "?", "texcoord", "material", "group", "rgba", "inflate", "subgrid"},
             {"geom", "?", "type", "contype", "conaffinity", "condim",
                 "group", "priority", "size", "material", "rgba", "friction", "mass",
-                "density", "solmix", "solref", "solimp", "margin", "gap", "surfacevel"},
+                "density", "solmix", "solref", "solimp", "margin", "gap", "surfacevel",
+                "adhesion"},
             {"site", "?", "group", "size", "material", "rgba"},
             {"plugin", "*", "plugin", "instance"},
             {"<"},
@@ -427,7 +429,7 @@ std::vector<const char*> MJCF[nMJCF] = {
     {"contact", "*"},
     {"<"},
         {"pair", "*", "name", "class", "geom1", "geom2", "condim", "friction",
-            "solref", "solreffriction", "solimp", "gap", "margin"},
+            "solref", "solreffriction", "solimp", "gap", "margin", "adhesion"},
         {"exclude", "*", "name", "body1", "body2"},
     {">"},
 
@@ -1975,6 +1977,7 @@ void mjXReader::OneGeom(XMLElement* elem, mjsGeom* geom) {
   ReadAttr(elem, "margin", 1, &geom->margin, text);
   ReadAttr(elem, "gap", 1, &geom->gap, text);
   ReadAttr(elem, "surfacevel", 6, geom->surfacevel, text, false, false);
+  ReadAttr(elem, "adhesion", 1, &geom->adhesion, text);
   if (ReadAttrTxt(elem, "hfield", hfieldname)) {
     mjs_setString(geom->hfieldname, hfieldname.c_str());
   }
@@ -2197,6 +2200,7 @@ void mjXReader::OnePair(XMLElement* elem, mjsPair* pair) {
   ReadAttr(elem, "solimp", mjNIMP, pair->solimp, text, false, false);
   ReadAttr(elem, "margin", 1, &pair->margin, text);
   ReadAttr(elem, "gap", 1, &pair->gap, text);
+  ReadAttr(elem, "adhesion", 1, &pair->adhesion, text);
   ReadAttr(elem, "friction", 5, pair->friction, text, false, false);
 
   // write error info
@@ -2785,6 +2789,7 @@ void mjXReader::OneComposite(XMLElement* elem, mjsBody* body, mjsFrame* frame, c
     ReadAttr(egeom, "margin", 1, &dgeom.margin, text);
     ReadAttr(egeom, "gap", 1, &dgeom.gap, text);
     ReadAttr(egeom, "surfacevel", 6, dgeom.surfacevel, text, false, false);
+    ReadAttr(egeom, "adhesion", 1, &dgeom.adhesion, text);
     if (ReadAttrTxt(egeom, "material", material)) {
       mjs_setString(dgeom.material, material.c_str());
     }