Expose AssemblyObject::getSolveContext() to Python and hook into the
.kc save flow so that silo/solver/context.json is packed into every
assembly archive. This lets server-side solver runners operate on
pre-extracted constraint graphs without a full FreeCAD installation.
Changes:
- Add public getSolveContext() to AssemblyObject (C++ and Python)
- Build Python dict via CPython C API matching kcsolve.SolveContext.to_dict()
- Register _solver_context_hook in kc_format.py pre-reinject hooks
- Add silo/solver/context.json to silo_tree.py _KNOWN_ENTRIES
