Add mjs_makeFlex to the MuJoCo C and Python APIs.

- Add mjs_makeFlex C API that wraps mjCFlexcomp::Make(), creating
  a flex with auto-generated bodies, joints, and equality constraints
  from parameters (type, dim, dof, count, cellcount, spacing, scale,
  radius, mass, inertiabox, equality, rigid, flatskin, elastic2d,
  pos, quat, origin, file, vfs).
- Add Python binding body.make_flex() in specs.cc with keyword args.
- Refactor flex_spec_provision.py to use make_flex() instead of XML
  string templating + separate spec + attach. Eliminates textwrap
  dependency.
- Merge user_flexcomp into user_model BUILD target to resolve
  circular dependency.

PiperOrigin-RevId: 922725304
Change-Id: Ice13787c45f3173407fec659f9590a6453bc65dd

doc/APIreference/functions.rst

@@ -4496,6 +4496,17 @@ Add sensor.
 
 Add flex.
 
+.. _mjs_makeFlex:
+
+`mjs_makeFlex <#mjs_makeFlex>`__
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. mujoco-include:: mjs_makeFlex
+
+Add flexcomp: create flex with auto-generated bodies/joints, return flex spec.
+
+*Nullable:* ``type``, ``dof``, ``count``, ``cellcount``, ``spacing``, ``scale``, ``pos``, ``quat``, ``origin``, ``file``, ``vfs``
+
 .. _mjs_addPair:
 
 `mjs_addPair <#mjs_addPair>`__

doc/changelog.rst

@@ -5,11 +5,16 @@ Changelog
 Upcoming version (not yet released)
 -----------------------------------
 
+General
+^^^^^^^
+- Added :ref:`mjs_makeFlex`, a new C API function equivalent to the :ref:`flexcomp<body-flexcomp>` element for
+  programmatically creating flex objects with auto-generated bodies, joints, and equality constraints. Exposed as
+  ``body.make_flex()`` in Python.
+
 Bug fixes
 ^^^^^^^^^
 - Fixed a bug in the ``mjz`` :ref:`decoder <mjpDecoder>` where unnormalized paths would fail to be read.
 
-
 Version 3.9.0 (May 27, 2026)
 ----------------------------
 

doc/includes/references.h

@@ -3678,6 +3678,12 @@ int mjs_delete(mjSpec* spec, mjsElement* element);
 mjsActuator* mjs_addActuator(mjSpec* s, const mjsDefault* def);
 mjsSensor* mjs_addSensor(mjSpec* s);
 mjsFlex* mjs_addFlex(mjSpec* s);
+mjsFlex* mjs_makeFlex(mjsBody* body, const char* name, const char* type, int dim,
+                      const char* dof, const int count[3], const int cellcount[3],
+                      const double spacing[3], const double scale[3], double radius,
+                      double mass, double inertiabox, int equality, int rigid, int flatskin,
+                      int elastic2d, const double pos[3], const double quat[4],
+                      const double origin[3], const char* file, const mjVFS* vfs);
 mjsPair* mjs_addPair(mjSpec* s, const mjsDefault* def);
 mjsExclude* mjs_addExclude(mjSpec* s);
 mjsEquality* mjs_addEquality(mjSpec* s, const mjsDefault* def);