Incorrect R,Z conversion in NetCDF output for MEISS/splined field

[krystophny]

Summary

When using MEISS (splined) field with chartmap coordinates, the xend_cart positions in the NetCDF output are incorrect because zend (integrator coordinates) is passed directly to ref_coords%evaluate_cart() without converting to reference coordinates first.

Root Cause

In src/netcdf_results_output.f90:326:

call ref_coords%evaluate_cart(zend(1:3, i), xend_cart(:, i))

The zend array contains coordinates in the integrator frame (after scaling and phi gauge transformation), but ref_coords%evaluate_cart() expects reference coordinates.

For MEISS field, the integrator coordinates differ from reference coordinates due to:

1. Radial scaling (identity_scaling or sqrt_s_scaling)
2. Phi gauge transformation (lam_phi)

The conversion integ_to_ref_meiss() in field_can_meiss.f90 handles this, but it is not called before writing NetCDF output.

Impact

• Particle footprint R,Z positions are incorrect for chartmap-based tracing
• Particles may appear to hit the wall at wrong locations (rho<1 inside LCFS)
• Heat flux maps based on xend_cart are spatially incorrect
• Loss fractions are correct (verified 57.32% match between VMEC and chartmap)

Fix

Add integ_to_ref conversion before evaluate_cart in netcdf_results_output.f90:

real(dp) :: zend_ref(3)
call integ_to_ref(zend(1:3, i), zend_ref)
call ref_coords%evaluate_cart(zend_ref, xend_cart(:, i))

Note: For CANFLUX/BOOZER, classification.f90 already converts zend angles to VMEC via can_to_vmec/boozer_to_vmec. This implicit conversion works because those field types don't have a phi gauge. MEISS is different.

Verification

After fix, chartmap footprint positions should match VMEC footprint positions for equivalent configurations.