Add GVEC QA Boozer chartmap validation

[krystophny]

Summary

Add GVEC QA comparison path on top of the real-Cartesian Boozer chartmap contract, with cleaned-up test infrastructure and GVEC documentation.

Production code (14 lines in src/boozer_converter.F90):

• Apply vmec_B_scale and vmec_RZ_scale to chartmap field data after loading from NetCDF

Test infrastructure:

• GVEC-to-Boozer-chartmap conversion for the shared QA case
• Strict roundtrip comparison against external GVEC chartmap
• End-to-end QA benchmarking (QA, QH, NCSX equilibria)
• Figure-8 stellarator golden record test
• Chartmap scaling unit test
• De-duplicated test helpers (run_cmd, plot utilities) into shared artifacts module
• Fixed double test_chartmap_scaling registration in CMakeLists.txt

Documentation:

• GVEC Boozer chartmap workflow section in README
• Example simple_chartmap.in configuration for chartmap mode
• Reference to tools/gvec_to_boozer_chartmap.py conversion tool
• Polished examples and README quickstart (cherry-picked from Polish examples and README quickstart #342)

Verification

All tests pass (52/52)

$ make CONFIG=Fast && make test
100% tests passed, 0 tests failed out of 52

Label Time Summary:
chartmap         =   0.10 sec*proc (1 test)
golden_record    =   6.32 sec*proc (1 test)
integration      = 312.46 sec*proc (18 tests)
performance      =   1.12 sec*proc (2 tests)
plot             = 315.41 sec*proc (7 tests)
python           = 532.90 sec*proc (9 tests)
slow             = 773.22 sec*proc (9 tests)
system           =  28.67 sec*proc (3 tests)
unit             =  44.49 sec*proc (12 tests)

Total Test time (real) = 912.88 sec

Test plan

• make test-fast passes (41/41 tests)
• make test passes (52/52 tests including slow)
• De-duplicated test helpers maintain identical behavior
• Python syntax validation on all modified test files
• README GVEC section documents complete workflow
• Example simple_chartmap.in matches test-verified configuration