feat: use real Cartesian Boozer chartmaps

[krystophny]

Summary

Switch the VMEC Boozer chartmap export to store the true Cartesian embedding and mark the toroidal parameter as zeta_convention="boozer".

This is the minimal SIMPLE-side contract change needed before the GVEC-backed validation layers.

Verification

cmake --build build -j$(nproc)
ctest --test-dir build --output-on-failure -R '^(test_boozer_chartmap_roundtrip|test_e2e_boozer_chartmap)$'

100% tests passed, 0 tests failed out of 2