Fix GVEC fetch URL and stabilize GVEC tests

[krystophny]

Summary

• switch the GVEC FetchContent URL to gvec-group/gvec
• disable libneo subproject tests during dependency setup so GVEC-enabled configure no longer collides on duplicate test targets
• restore the GVEC theta*/covariant component transform used by SIMPLE's direct field comparison
• fix the GVEC tests to use binary-dir fixtures, build the gvec executable when needed, compare VMEC and GVEC on the same radial convention, and use a non-coarse VMEC spline order for the tight comparison

Verification

$ make configure BUILD_DIR=/tmp/simple-gvec-build2 FLAGS='-DENABLE_GVEC=ON -DENABLE_PYTHON_INTERFACE=OFF -DSIMPLE_ENABLE_PYTHON_TOOLS=OFF -DPython_EXECUTABLE=/usr/bin/python3'
$ cmake --build /tmp/simple-gvec-build2 --target test_gvec.x test_vmec_gvec.x test_canonical_gvec.x test_adapter_consistency.x test_vmec_gvec_adapter.x test_simple_vmec_gvec.x -j$(nproc)
$ ctest --test-dir /tmp/simple-gvec-build2/test --output-on-failure -R '^(test_gvec|test_vmec_gvec|test_canonical_gvec|test_adapter_consistency|test_vmec_gvec_adapter|test_simple_vmec_gvec)$'
Test project /tmp/simple-gvec-build2/test
1/6 Test #4: test_gvec ........................   Passed    0.58 sec
2/6 Test #5: test_vmec_gvec ...................   Passed    2.67 sec
3/6 Test #6: test_canonical_gvec ..............   Passed    0.45 sec
4/6 Test #7: test_simple_vmec_gvec ............   Passed    0.05 sec
5/6 Test #8: test_adapter_consistency .........   Passed    0.43 sec
6/6 Test #9: test_vmec_gvec_adapter ...........   Passed    0.60 sec

100% tests passed, 0 tests failed out of 6

[krystophny]

Closing this PR because it conflicts with the GVEC removal that is now on main. Rebasing it would reintroduce GVEC files and tests that were intentionally deleted in PR #328, so the current branch is no longer the right vehicle for this work. If any part of the fetch/test work is still needed, it should be respun from current main without reviving the removed GVEC path.