Add periodic-vs-free validation test

[sergioald]

Summary

Adds a dedicated validation test comparing the periodic flow solver against the free-flow solver for a spatially periodic curvature field.

Changed

• tests/test_periodic_vs_free_convergence.py
  • solves the same periodic curvature forcing with parall_u_periodic(...) and parall_u_free(...);
  • compares the interior free-flow response against the periodic response on long domains;
  • verifies that agreement improves as the free-flow domain length increases;
  • checks that both solvers agree in shape on adequate domains;
  • checks that the periodic solver returns zero velocity perturbation for zero curvature.

Why

The periodic boundary-condition path has been identified as needing dedicated validation against a trusted reference.

For a spatially periodic curvature field, the periodic solution should match the far-field/interior limit of the free-flow solution when the free-flow domain is sufficiently long. This PR adds that validation as a regression test rather than relying only on solver smoke tests.

The test deliberately checks convergence with increasing domain length, because shorter domains can still be dominated by inlet/outlet effects and are not a meaningful periodic-reference comparison.

Validation

python -m py_compile tests/test_periodic_vs_free_convergence.py
python -m pytest tests/test_periodic_vs_free_convergence.py
python -m pytest
python -m run_ldsfl --base-dir . --cases 1 --max-steps 100 --no-plots

Notes

This is a tests-only PR.

It does not change the morphodynamic update algorithm, flow solvers, curvature calculation, stability diagnostics, CLI interface, GUI interface, output naming, or generated output format.