Improve coordinate scaling documentation in code

[krystophny]

Priority: Medium

Problem

The coordinate scaling system (sqrt_s_scaling vs identity_scaling) requires careful Jacobian handling that is easy to get wrong. The relationship between different radial coordinates:

• s = normalized toroidal flux (VMEC native)
• rho = sqrt(s) = common radial coordinate
• Jacobian factor: d(rho)/ds = 1/(2*sqrt(s))

When switching between scalings or adding new coordinate systems, it is easy to miss the required Jacobian adjustments.

Current State

• Code works correctly but the scaling logic is spread across multiple files
• Comments explaining the transformations are sparse
• New developers may introduce bugs when modifying coordinate handling

Proposed Solution

1. Add detailed comments at key transformation points in:
   • field_splined.f90 (spline construction)
   • reference_coordinates.f90 (scaling selection)
   • libneo coordinate implementations
2. Consider adding validation/assertions for coordinate ranges
3. Document the full transformation chain in code comments

Related

• See DOC/coordinates-and-fields.md Section 2.4 and Appendix A.2