Require FreeGS4E as the equilibrium backend

[nicamo]

Summary

• make FreeGS4E a required runtime dependency instead of an optional extra
• simplify installation and CI commands accordingly
• clarify that FreeGSNKE documentation defines the supported API
• preserve and document editable FreeGS4E co-development

This is the FreeGSNKE-side companion to FusionComputingLab/freegs4e#62, which clarifies FreeGS4E's supported backend scope and validates candidate changes against FreeGSNKE.

Validation

• built wheel declares freegs4e>=0.14,<0.15 as a required dependency
• obsolete freegs4e extra is absent from wheel metadata
• FreeGSNKE against the candidate FreeGS4E branch: 64 passed, 4 skipped