Refactor flex strain constraints to be per-cell.

Each mjEQ_FLEXSTRAIN equality now represents a single cell within a flex. This allows for more efficient sparse Jacobian computation by only considering the degrees of freedom of the nodes within each specific cell. This change gives a speedup of about 10x on a 3x3x3 model.

PiperOrigin-RevId: 902164069
Change-Id: I78eedf1d5cf39b8989fe9863c22d164922fc0efb

doc/XMLreference.rst

@@ -4888,8 +4888,11 @@ constraint type is only supported for dimension 3 flexes with trilinear or quadr
 :at:`flex`: :at-val:`string, required`
    Name of the flex whose strain is being constrained.
 
+.. _equality-flexstrain-cell:
 
-
+:at:`cell`: :at-val:`int(3), optional`
+   3D grid index (i, j, k) identifying the cell in the flex object. The grid size is specified in the :ref:`cellcount
+   <deformable-flex-cellcount>` attribute.
 
 
 .. _tendon:

doc/XMLschema.rst

@@ -2062,6 +2062,9 @@
             .. grid-item::
                :ref:`flex<equality-flexstrain-flex>`
 
+            .. grid-item::
+               :ref:`cell<equality-flexstrain-cell>`
+
             .. grid-item::
                :ref:`active<equality-flexstrain-active>`
 

doc/changelog.rst

@@ -10,6 +10,9 @@ General
 
 - Added :ref:`multi-cell support<body-flexcomp-cellnum>` for trilinear and quadratic flexes. Note that the implicit
   integrator uses a dense solver for the flex degrees of freedom, which can be slow for multi-cell flexes.
+- Refactored ``flexstrain`` equality constraints to be instantiated per cell instead of per flex object, reducing the
+  number of degrees of freedom per constraint row. The equality can be associated with a specific cell with the new
+  attribute ":ref:`cell <equality-flexstrain-cell>`
 
 Version 3.7.0 (April 14, 2026)
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