Implement multi-cell finite element method for interpolated flexes.

This change introduces a `flex_cellcount` field to `mjModel` to specify the number of cells in each dimension for interpolated flexes. The stiffness computation, passive force calculation, and Jacobian derivatives are updated to operate on a per-cell basis, significantly improving performance by localizing computations to the nodes within each cell.

PiperOrigin-RevId: 901216393
Change-Id: Ic23132e609de11e71bb7fef8d1f139daad2ec264

doc/XMLschema.rst

@@ -1417,6 +1417,9 @@
             .. grid-item::
                :ref:`count<body-flexcomp-count>`
 
+            .. grid-item::
+               :ref:`cellcount<body-flexcomp-cellcount>`
+
             .. grid-item::
                :ref:`spacing<body-flexcomp-spacing>`
 
@@ -1657,6 +1660,12 @@
             .. grid-item::
                :ref:`node<deformable-flex-node>`
 
+            .. grid-item::
+               :ref:`cellcount<deformable-flex-cellcount>`
+
+            .. grid-item::
+               :ref:`dof<deformable-flex-dof>`
+
 
          .. dropdown:: :ref:`contact<flex-contact>` :octicon:`dot`