Fix field_eq flux gauge and translated wall stretching

[krystophny]

Summary

• normalize field_eq poloidal-flux state to one shifted gauge and one cgs unit
• make convex-wall stretching invariant under rigid vertical translation
• add focused native regressions for both defects

Root causes

field_eq converted and shifted psif and psi_sep to G cm²/rad while leaving psi_axis in the GEQDSK input unit and absolute gauge. Downstream normalized-flux expressions then subtracted incompatible quantities.

Separately, stretch_coords inferred a radial wall center but hard-coded the vertical center to Z=0 in its polar map. A vertically displaced equilibrium could therefore map interior points outside and change under a rigid Cartesian translation.

Changes

• serialize psi_axis=0, psif=(PSIRZ-SIMAG)*1e8, and psi_sep=(SIBRY-SIMAG)*1e8
• retain the existing magnetic-field components while making normalized flux gauge invariant
• infer both R0 and Z0 from the convex wall and consistently use local (R-R0,Z-Z0) coordinates
• regress axis/LCFS flux endpoints and an additive GEQDSK flux-gauge shift
• regress a translated circular wall, including interior/exterior classification and ray preservation

Validation

• all 10 field-labelled native tests pass
• the translated-wall oracle fails on the old hard-coded-Z=0 implementation and passes after the fix
• the flux-endpoint regression verifies one shifted G cm²/rad gauge at the magnetic axis and LCFS

Relationships

Fixes #401.
Fixes #402.

This is a stacked dependency of itpplasma/NEO-RT#80. It changes flux metadata/state and translated-wall geometry; it does not infer COCOS, flip a field component, or fit a cross-code sign, phase, gain, or radius.

[krystophny]

Reviewed latest b5272f4d after merging current main. The shifted G cm^2/rad flux gauge is consistent at axis and LCFS, while wall stretching now derives both R and Z centers and is translation invariant. These are behavioral endpoint/translation oracles and do not infer COCOS or flip field components. Native suite: 95/95 passed. No blocking finding.