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

test/user/user_composite_test.cc

@@ -72,6 +72,26 @@ TEST_F(UserCompositeTest, GeomSurfacevel) {
   }
 }
 
+TEST_F(UserCompositeTest, GeomAdhesion) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+  <worldbody>
+    <composite type="cable" count="5 1 1" curve="s">
+      <geom type="capsule" size=".02" adhesion="2"/>
+    </composite>
+  </worldbody>
+  </mujoco>
+  )";
+  std::array<char, 1024> error;
+  MjModelPtr m = LoadModelFromString(xml, error.data(), error.size());
+  ASSERT_THAT(m.get(), NotNull()) << error.data();
+  for (int g = 0; g < m->ngeom; g++) {
+    if (m->geom_type[g] == mjGEOM_CAPSULE) {
+      EXPECT_EQ(m->geom_adhesion[g], 2);
+    }
+  }
+}
+
 TEST_F(UserCompositeTest, InvalidShape) {
   static constexpr char xml[] = R"(
   <mujoco>