Preserve blank GUI equivalence transient step

[sergioald]

Summary

Preserves a blank GUI equivalence transient-step field when saving GUI configs.

Changed

• gui_ldsfl.py

  • adds _raw_tk_string(...) for config-saving paths where blank values are meaningful;
  • changes saved gui_state["sinuo_equiv_transient_step"] to preserve the raw field value instead of falling back to "40000".

• tests/test_gui_sinuosity_metrics.py

  • documents that blank transient-step values are preserved by the raw getter;
  • keeps _safe_tk_string(...) behaviour unchanged for live diagnostic defaults.

Why

Codex correctly flagged that PR #37 saved a blank “Equiv. transient step” GUI field as "40000".

In this field, blank is meaningful: _build_config() converts it to None, which requests all-history equivalence analysis. Saving it as "40000" means that loading the saved GUI config would silently change the diagnostic window back to post-40000-only analysis.

This follow-up preserves the user’s blank value during config saving while keeping the safe defaulting behaviour for live diagnostic calculations.

Validation

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

Notes

This is a targeted PR #37 follow-up.

It does not change the solver algorithm, flow solvers, curvature calculation, stability estimator mathematics, CLI interface, output naming, fixture structure, or generated output format.