[gfleming1992]

Change

OCP.StepKinematics is absent from the generated bindings even though OCCT provides these STEP entities. Add StepKinematics to the module list so Python clients can author and inspect standard kinematic links, joints, and pairs.

Add a regression test that creates a revolute pair and a gear pair, writes them through the existing STEP exchange API, reloads them, and checks their concrete types, linked names, rotational flags, radii, and gear ratio. A transferred vertex initializes the writer session before adding the supplemental entities.

This exposes OCCT data entities; it does not add a motion solver or claim a complete geometry-bound mechanism exchange workflow.

Validation

• Generated and compiled the bindings against OCCT 8.0.1 on macOS arm64 with Python 3.12.
• PYTHONPATH=<compiled OCP> python -m pytest tests -q: 18 passed.
• Local generation used a checkpointing modification to pywrap and local macOS CMake search-path settings. Those build-environment changes are excluded from this PR; upstream CI still needs to verify the standard build and other platforms.