Consume pyQSC_JAX external 3+5+7 field jets

[rogeriojorge]

Summary

• add a generic JAX magnetic-field Hessian interface;
• consume pyQSC_JAX total-vacuum or surface-free external field jets;
• use smooth, dimensionally normalized 3+5+7 least-squares residual blocks;
• verify exact vacuum reduction and finite-beta external-target subtraction;
• differentiate through real coil shapes/currents and pyQSC_JAX
  axis/physics variables;
• add executable finite-beta stage-two and single-stage optimization
  examples.

Pressure-only stellarator examples

Both demonstrations now use public database-backed
plasma_stellarator (Wisconsin database ID 52521):

• finite p2 and exactly I2 = 0;
• nonplanar axis with nonzero torsion;
• |iota| > 2.8;
• formal radius 0.15 m;
• normalized field, STF-gradient, and STF-Hessian residual blocks.

The single-stage design vector is (rc1, zs1, etabar, B2c); toroidal current
is fixed at zero and cannot drift toward a tokamak-like solution. Saved
metadata include the complete Fourier axis and database provenance.

Validation

• 13 focused tests pass, including both optimization scripts in clean
  subprocesses.
• The pressure-only regression checks finite pressure, exactly zero current,
  |iota| > 0.4, RMS torsion above 0.5 1/m, more than 10% angular
  variation in the plasma-field norm, and nonzero field/Hessian subtraction.
• Stage two: normalized objective 6.7102e7 -> 4.4677e3; field, gradient,
  and Hessian block sums independently decrease
  4.4786 -> 0.7913, 2.0187e4 -> 72.3078, and
  2.6833e8 -> 1.7578e4.
• Single stage: objective 6.7102e7 -> 3.0052e4; final blocks are
  0.6382, 137.638, and 1.1966e5; weighted B20 residual
  0.2930 -> 0.2514; final |iota| = 2.843, I2 = 0, and RMS torsion
  0.856 1/m.
• Coil-shape/current and near-axis gradients agree with finite differences;
  vacuum reduction is exact.

These deliberately short runs are execution and coupled-gradient
demonstrations, not claims of device-quality global optima.

Dependency and inherited state

This draft is paired with
uwplasma/pyQSC_JAX#2;
the final boundary/VMEX hardening source commit is 060aef3. Dependency
direction remains ESSOS -> pyQSC_JAX.

The ESSOS base branch retains unrelated collection, formatting, and
documentation failures already present before this patch. They remain
disclosed rather than being mixed into the field-jet integration. No
generated optimization output is committed.

This PR should remain draft until the paired pyQSC_JAX API lands.