Add geom surfacevel: zero-dof conveyors, treadmills and turntables.

https://www.youtube.com/watch?v=PdSdrqhSiZA

The new geom attribute surfacevel (6 numbers: linear and angular velocity in the geom's local frame, angular about the geom frame origin) specifies the velocity of the geom's surface material relative to the geom frame. The relative surface velocity of the two geoms is added to the tangential contact rows of efc_vel in mj_referenceConstraint, so friction drives touching bodies toward the motion of the surface: objects on a conveyor are transported at belt speed, turntables impart omega x r with torsional spin-up for condim >= 4, and surface velocities compose with each other and with body motion. The component along the contact normal is projected out: probe experiments showed that velocity-space emission chatters mass-independently and ingestion merely deepens penetration; normal-direction effects belong to force-space features.

surfacevel is interpreted in the geom frame as authored: for mesh geoms, whose compiled frame absorbs the mesh centering and principal-axes transform, the compiler re-expresses the authored value in the compiled frame.

No special interaction with sleeping: objects being transported do not fall asleep because they are moving; objects at rest on an active surface may sleep like any other resting object.

Includes showcase models (model/surfacevel/): a luggage carousel whose ring is a spinning square-profile supertorus fed by a cascade of belts with matched spinning end rollers, bags dropping in and circulating indefinitely; and a treadmill with a passive humanoid.

PiperOrigin-RevId: 948647785
Change-Id: I0c6559a91cc7ece1237eb8ac2e51986e7342d962

doc/XMLreference.rst

@@ -2771,6 +2771,32 @@ helps clarify the role of bodies and geoms in MuJoCo.
    actuators which use contacts in the gap zone to generate adhesive forces without producing contact forces.
    See :ref:`margin and gap<coMarginGap>`.
 
+.. _body-geom-surfacevel:
+
+.. youtube:: PdSdrqhSiZA
+   :align: right
+   :width: 35%
+
+:at:`surfacevel`: :at-val:`real(6), "0 0 0 0 0 0"`
+   Velocity of the geom's surface as seen by contacts, given as a velocity field :math:`\sigma(x)` with two components:
+   a constant velocity :math:`v` (first three numbers) and a rotational field with angular velocity :math:`\omega` (last
+   three numbers) about the geom frame origin :math:`p`, both expressed in the geom frame:
+
+   .. math::
+      \sigma(x) = v + \omega \times (x - p)
+
+   A contact with the geom observes the surface moving along this field, with the velocity projected onto the contact's
+   tangent plane: no normal velocity is imparted. When :at:`condim` is 4 or larger, the angular velocity :math:`\omega`
+   also drives torsional friction. :at:`surfacevel` models surfaces that move while the geom itself does not: conveyor
+   belts, treadmills and turntables can be constructed with no degrees of freedom. Friction drives touching bodies along
+   the motion of the surface: objects placed on a conveyor are transported at belt speed, and a turntable (angular
+   surface velocity about the cylinder axis) imparts tangential velocity that grows with radius. Surface velocities of
+   two touching geoms compose as relative velocity, and compose correctly with body motion (a conveyor mounted on a
+   moving vehicle works as expected). Note that this attribute describes the geom's *entire* surface: a box with
+   constant ``surfacevel`` moves all six faces. When contact points are visualized, a contact with a moving surface
+   additionally displays an arrow along the tangential surface velocity at the contact point. This attribute can be
+   modified at runtime.
+
 .. _body-geom-fromto:
 
 :at:`fromto`: :at-val:`real(6), optional`
@@ -3460,10 +3486,12 @@ joints and tendons have different sets of attributes, while all geoms in the com
 
 .. _composite-geom-gap:
 
+.. _composite-geom-surfacevel:
+
 .. |body/composite/geom attrib list| replace::
    :at:`type`, :at:`contype`, :at:`conaffinity`, :at:`condim`, :at:`group`, :at:`priority`, :at:`size`, :at:`material`,
    :at:`rgba`, :at:`friction`, :at:`mass`, :at:`density`, :at:`solmix`, :at:`solref`, :at:`solimp`, :at:`margin`,
-   :at:`gap`
+   :at:`gap`, :at:`surfacevel`
 
 |body/composite/geom attrib list|
    Same meaning as regular :ref:`geom <body-geom>` attributes.
@@ -9540,6 +9568,8 @@ if omitted.
 
 .. _default-geom-gap:
 
+.. _default-geom-surfacevel:
+
 .. _default-geom-fromto:
 
 .. _default-geom-axisangle: