Free-boundary equilibrium with FreeGS-compatible coil model

[anulum]

Goal

Extend the Grad-Shafranov solver from fixed-boundary to free-boundary equilibrium.

Scope

• FreeGS-compatible coil geometry model
• Inverse reconstruction mode (fit to magnetic probe measurements)
• Benchmark: NRMSE < 5% against EFIT on 10+ shots

Current state

GS solver uses fixed-boundary (coil currents as inputs). FreeGS strict parity
lane exists (.github/workflows/freegs-strict.yml) but tests synthetic cases only.

Acceptance criteria

• Free-boundary solver converges on SPARC-like geometry
• Inverse mode reconstructs from probe signals
• NRMSE < 5% vs EFIT on 10+ shots
• FreeGS strict CI lane passes

Roadmap: v4.0