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/XMLreference.rst

@@ -3700,8 +3700,14 @@ saving the XML:
 .. _body-flexcomp-count:
 
 :at:`count`: :at-val:`int(3), "10 10 10"`
-   The number of automatically generated points in each dimension. This and the next attribute only apply to types grid,
-   box, cylinder, ellipsoid.
+   Specifies the number of automatically generated points in each dimension for types **grid**, **box**, **cylinder**,
+   and **ellipsoid**.
+
+.. _body-flexcomp-cellcount:
+
+:at:`cellcount`: :at-val:`int(3), "1 1 1"`
+   Specifies the number of cells in each dimension for the background interpolation grid when using **trilinear** or
+   **quadratic** dofs.
 
 .. _body-flexcomp-spacing:
 
@@ -4242,6 +4248,17 @@ cases, the user will specify a :el:`flexcomp` which will then automatically cons
    An array of MuJoCo body names (separated by white space) to which each node belongs. The number of body names
    should equal the number of nodes (nnode). See the flexcomp :ref:`dof<body-flexcomp-dof>` attribute for more details.
 
+.. _deformable-flex-cellcount:
+
+:at:`cellcount`: :at-val:`int(3), optional`
+   When using **trilinear** or **quadratic** dofs, this specifies the number of cells in each dimension for the
+   background interpolation grid.
+
+.. _deformable-flex-dof:
+
+:at:`dof`: :at-val:`[trilinear, quadratic], optional`
+   Interpolation order for the flex.
+
 .. _flex-edge:
 
 :el-prefix:`flex/` |-| **edge** |?|