Inconsistent sqgBctr support across field types

[krystophny]

Priority: Medium

Problem

The sqgBctr (sqrt(g) * B contravariant) output is inconsistently supported across field types:

Field Type	sqgBctr Support
vmec_field_t	Not implemented (returns 0)
coils_field_t	Returns raw Cartesian B
gvec_field_t	Fully implemented
splined_field_t	Not implemented

This inconsistency can cause subtle bugs when switching between field types or when integrators assume sqgBctr is available.

Proposed Solution

Either:

1. Implement consistently: Add sqgBctr computation to all field types
2. Document clearly: Add runtime checks and clear documentation about which integrators require sqgBctr

Related

• See DOC/coordinates-and-fields.md Appendix A.5