run_fem_analysisRun the CalculiX solver on an existing `Fem::FemAnalysis` and return summary results (max von Mises stress, max displacement, node count, working directory). Auto-creates a `SolverCcxTools` if the analysis has none. See [`examples/cantilever_fem.py`](examples/cantilever_fem.py) for an end-to-end usage example.