Finish GVEC straight-fieldline support end-to-end and add PEST

[krystophny]

Summary

The current GVEC export-based integration is in good shape for:

• GVEC fetch/build
• logical-coordinate export + refcoords path
• Boozer export generation
• direct Boozer field comparison
• Boozer refcoords magfie comparison

What is still open is the remaining straight-fieldline end-to-end path and the still-missing PEST family.

Current Status

Passing locally:

ctest --test-dir /tmp/simple-gvec-reset-build/test --output-on-failure \
  -R '^(generate_gvec_boozer_export|test_gvec_boozer|test_gvec_boozer_refcoords_magfie)$'

Still failing locally:

ctest --test-dir /tmp/simple-gvec-reset-build/test --output-on-failure \
  -R '^(generate_gvec_export|test_simple_vmec_gvec)$'

Current failure:

ERROR STOP Maximum steps exceeded in ODE integration
...
#5  __alpha_lifetime_sub_MOD_integrate_mfl_can at src/sub_alpha_lifetime_can.f90:320
#6  __samplers_MOD_init_starting_surf at src/samplers.f90:41
#7  __simple_main_MOD_main at src/simple_main.f90:100

Problem Statement

The remaining GVEC gap is no longer the raw export reader or direct Boozer field evaluation. The unresolved part is the end-to-end straight-fieldline workflow used by SIMPLE during starting-surface construction and full particle runs.

That path still is not fully aligned with the new export-based GVEC interface:

• the Boozer start-surface path still fails in test_simple_vmec_gvec
• the generic starting-surface ODE path is still the place where the GVEC straight-fieldline integration breaks down
• PEST is not implemented yet even though the requirement explicitly calls for logical or straight fieldline (Boozer or PEST)

Scope

1. Finish GVEC Boozer support end-to-end so the full test_simple_vmec_gvec workflow passes.
2. Add PEST export + read + test coverage on the same abstraction layer.
3. Keep the current no-legacy design: only the NetCDF export interface, no raw GVEC .dat fallback, no VMEC shim path.

Concrete Tasks

• Make the starting-surface construction work for GVEC straight-fieldline coordinates.
  • Either finish the special-case straight-fieldline sampling path cleanly, or make the generic path robust enough for exported GVEC Boozer data.
  • The result must remove the current integrate_mfl_can failure in the Boozer refcoords run.
• Audit the scaled/native radial-coordinate contract for refcoords sampling and orbit setup.
  • The straight-fieldline GVEC path must use the same first-coordinate convention consistently in magfie_refcoords, samplers, and the export reader.
• Ensure the straight-fieldline path consumes the exact exported field-line data where appropriate.
  • In particular, the end-to-end path must not silently fall back to reconstructed or spline-induced approximations when exact exported contravariant information is available.
• Implement PEST export support in python/pysimple/gvec_export.py.
• Extend the Fortran GVEC readers and reference-coordinate logic so PEST files are detected and loaded through the same modern interface.
• Add tests that cover the currently missing behavior.

Required Tests

• Keep these passing:
  • test_gvec
  • test_vmec_gvec
  • test_gvec_boozer
  • test_gvec_boozer_refcoords_magfie
  • test_refcoords_file_detection
• Make this pass:
  • test_simple_vmec_gvec
• Add new PEST coverage:
  • export generation
  • file detection / refcoords initialization
  • direct field comparison
  • at least one end-to-end SIMPLE run

Acceptance Criteria

• SIMPLE can use GVEC logical coordinates end-to-end.
• SIMPLE can use GVEC Boozer coordinates end-to-end, including the current test_simple_vmec_gvec case.
• SIMPLE can read a GVEC PEST export through the same NetCDF interface.
• Unit, integration, and system/E2E tests cover logical + Boozer + PEST without regressions in VMEC or chartmap paths.
• No legacy GVEC interfaces are reintroduced.

Relevant Files

• python/pysimple/gvec_export.py
• src/gvec/gvec_export_data.f90
• src/coordinates/gvec_reference_coordinates.f90
• src/field/field_gvec.f90
• src/magfie.f90
• src/samplers.f90
• src/simple_main.f90
• test/tests/test_simple_vmec_gvec.f90
• test/tests/test_gvec_boozer.f90
• test/tests/test_gvec_boozer_refcoords_magfie.f90

Notes

This is the remaining follow-up after the current GVEC export refactor work on fix/gvec-fetch-and-tests / PR #325.