feat: support real-Cartesian Boozer chartmaps

[krystophny]

Summary

Support real-Cartesian Boozer chartmaps in libneo by treating zeta_convention='boozer' as a genuine chart parameter instead of requiring atan2(y,x)=zeta.

This keeps the current fast cyl and vmec path intact while enabling a generalized chartmap path for GVEC-style Boozer files where the toroidal parameter is not the cylindrical angle.

What changed

• accept zeta_convention='boozer' in chartmap validation and metadata
• keep the atan2(y,x)=zeta contract only for cyl and vmec
• allow chartmap initialization for boozer
• spline the stored Cartesian geometry periodically in both angular directions for the generalized path
• route VMEC-to-chartmap mapping through from_cart(...) for boozer
• add dedicated validator and coordinate roundtrip tests for a non-cylindrical Boozer chartmap

Verification

cmake --build build -j$(nproc) --target test_chartmap_validator.x test_chartmap_boozer_coordinates.x test_chartmap_coordinates.x test_chartmap_vmec_mapping.x
ctest --test-dir build --output-on-failure -R 'test_chartmap_(validator|boozer_coordinates|coordinates|vmec_mapping)$'

Result:

• 100% tests passed, 0 tests failed out of 4

Integration note

The same commit is also pushed to feature/boozer-chartmap in this repo so SIMPLE PR 327 can auto-resolve the matching libneo branch in CI.

[krystophny]

Superseded by PR #261 so both repos use the single branch name . Closing this PR and deleting the extra branch.