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

python/mujoco/introspect/structs.py

@@ -2668,6 +2668,14 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  doc='interpolation (0: vertex, 1: nodes)',
                  array_extent=('nflex',),
              ),
+             StructFieldDecl(
+                 name='flex_cellnum',
+                 type=PointerType(
+                     inner_type=ValueType(name='int'),
+                 ),
+                 doc='finite cell num per dimension',
+                 array_extent=('nflex', 3),
+             ),
              StructFieldDecl(
                  name='flex_nodeadr',
                  type=PointerType(
@@ -8237,6 +8245,19 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  type=ValueType(name='int'),
                  doc='2D passive forces; 0: none, 1: bending, 2: stretching, 3: both',  # pylint: disable=line-too-long
              ),
+             StructFieldDecl(
+                 name='cellcount',
+                 type=ArrayType(
+                     inner_type=ValueType(name='int'),
+                     extents=(3,),
+                 ),
+                 doc='grid cell count for finite cell method',
+             ),
+             StructFieldDecl(
+                 name='order',
+                 type=ValueType(name='int'),
+                 doc='interpolation order (1: trilinear, 2: quadratic)',
+             ),
              StructFieldDecl(
                  name='nodebody',
                  type=PointerType(