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 paths intact while enabling the generalized chartmap path needed 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

SIMPLE integration

This is the libneo dependency for the stacked SIMPLE series ending in PR 327:

• SIMPLE PR 329: core SIMPLE Boozer chartmap path
• SIMPLE PR 330: roundtrip and VMEC benchmark validation
• SIMPLE PR 331: GVEC converter and figure-8 validation
• SIMPLE PR 327: real-Cartesian Boozer chartmaps

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

Review

Primary libneo/chartmap review: @krystophny
GVEC-use-case review: @Rykath

[krystophny]

@Rykath this is the libneo dependency for the stacked SIMPLE Boozer-chartmap series. The SIMPLE side is now split as PRs 329, 330, 331, and 327; this libneo PR is the generalized real-Cartesian Boozer chartmap piece underneath them.