Refactor flex stiffness storage to support variable sizes.

This change introduces `nflexstiffness` and `flex_stiffnessadr` to allow flex stiffness matrices to have sizes other than the fixed 21 per element. Higher-order flexes can now store larger stiffness matrices based on their number of nodes. The `flex_stiffnessadr` array provides the starting index for each flex's stiffness data within the `flex_stiffness` array.

PiperOrigin-RevId: 899632263
Change-Id: Ie49182c46c3777acf0c6b492345edfcf62fb5e44

python/mujoco/introspect/structs.py

@@ -972,6 +972,11 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  type=ValueType(name='mjtSize'),
                  doc='number of element vertex ids in all flexes',
              ),
+             StructFieldDecl(
+                 name='nflexstiffness',
+                 type=ValueType(name='mjtSize'),
+                 doc='number of stiffness parameters in all flexes',
+             ),
              StructFieldDecl(
                  name='nflexelemedge',
                  type=ValueType(name='mjtSize'),
@@ -2735,6 +2740,14 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  doc='first element vertex id address',
                  array_extent=('nflex',),
              ),
+             StructFieldDecl(
+                 name='flex_stiffnessadr',
+                 type=PointerType(
+                     inner_type=ValueType(name='int'),
+                 ),
+                 doc='stiffness matrix address',
+                 array_extent=('nflex',),
+             ),
              StructFieldDecl(
                  name='flex_elemedgeadr',
                  type=PointerType(
@@ -2965,7 +2978,7 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                      inner_type=ValueType(name='mjtNum'),
                  ),
                  doc='finite element stiffness matrix',
-                 array_extent=('nflexelem', 21),
+                 array_extent=('nflexstiffness',),
              ),
              StructFieldDecl(
                  name='flex_bending',