Add beta_p/li profile targets to pulse design example

[nicamo]

Summary

• add a Lao85 beta_p/li-to-alpha/beta fitting helper for static and evolutive workflows
• allow the nonlinear stepper to accept beta_p/li profile targets with cached metric-Jacobian reuse
• add example11b showing the pulse design workflow with constant beta_p/li targets and a tracking plot
• add regression tests for the fitter, nonlinear conversion, and cached linearized update path

Verification

• pytest freegsnke/tests/test_jtor_update.py -q
• python -m py_compile freegsnke/jtor_update.py freegsnke/nonlinear_solve.py freegsnke/equilibrium_update.py
• executed examples/example11b - pulse_design_tool_betap_li.ipynb top-to-bottom with nbconvert