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

unity/Runtime/Bindings/MjBindings.cs

@@ -1118,6 +1118,7 @@ public unsafe struct mjModel_ {
   public UInt64 nbuffer;
   public byte flg_gravcomp;
   public byte flg_surfacevel;
+  public byte flg_adhesion;
   public mjOption_ opt;
   public mjVisual_ vis;
   public mjStatistic_ stat;
@@ -1223,6 +1224,7 @@ public unsafe struct mjModel_ {
   public double* geom_margin;
   public double* geom_gap;
   public double* geom_surfacevel;
+  public double* geom_adhesion;
   public double* geom_fluid;
   public double* geom_user;
   public float* geom_rgba;
@@ -1444,6 +1446,7 @@ public unsafe struct mjModel_ {
   public double* pair_solimp;
   public double* pair_margin;
   public double* pair_gap;
+  public double* pair_adhesion;
   public double* pair_friction;
   public int* exclude_signature;
   public int* eq_type;
@@ -1624,6 +1627,7 @@ public unsafe struct mjContact_ {
   public fixed double solref[2];
   public fixed double solreffriction[2];
   public fixed double solimp[5];
+  public double adhesion;
   public double mu;
   public fixed double H[36];
   public int dim;
@@ -5801,6 +5805,7 @@ public unsafe struct mjData_ {
   public double* qfrc_damper;
   public double* qfrc_gravcomp;
   public double* qfrc_fluid;
+  public double* qfrc_adhesion;
   public double* qfrc_passive;
   public double* subtree_linvel;
   public double* subtree_angmom;