boozer chartmap: fix rho-grid fields and start sampling

[krystophny]

Summary

Fix the Boozer chartmap radial contract end to end.

Changes:

• A_phi, B_theta, B_phi, and Bmod are read on the chartmap rho grid. Radial derivatives are converted to s = rho^2 by chain rule.
• Chartmap files now carry and restore rmajor; both chartmap field consumers use boozer_chartmap_io.read_boozer_chartmap.
• Bmod is read on the endpoint-included theta_field/zeta_field grid.
• Chartmap startmode=1 now stores sampled starts in reference coordinates. The tracer returns integrator coordinates (s, theta_B, phi_B); chartmap zstart stores (rho, theta_B, phi_B), so sbeg=0.5 now writes rho=sqrt(0.5).
• Chartmap generate_start_only now stops after writing corrected starts instead of tracing orbits.
• Grid starts use the same conversion, and their linear index stride is fixed.
• Duplicate chartmap diagnostics were removed. The synthetic test_chartmap_aphi_abscissa now covers reader metadata and the analytic A_phi(rho) chain-rule contract.
• Chartmap tolerances were tightened to measured floors. The roundtrip Bmod floor is the current export resolution, about 7.90e-5; the orbit floor is about 1.68e-9.

Closes #358 and fixes the chartmap sbeg issue tracked in #359.

Stack

Base: main.

Merge order:

1. Merge this PR first.
2. Restack GVEC QA Boozer chartmap validation #334 after this PR. Do not merge GVEC QA Boozer chartmap validation #334 as it stands; its branch is based before several commits that are already on main and before this fix.

Already on main: #329, #332, #333, #338, #340, #346, #348, #355, and the Boozer flux-sign doc commit.

Verification

Test fails before the start-sampling fix

$ OMP_NUM_THREADS=1 ctest --test-dir build -R '^test_chartmap_startmode1$' --output-on-failure -j1
AssertionError: chartmap startmode=1 wrote s instead of rho
0% tests passed, 1 tests failed out of 1

Tests pass after the fix

$ cmake --build build --target simple.x -j2
[25/26] Linking Fortran executable simple.x

$ OMP_NUM_THREADS=1 ctest --test-dir build -R '^test_chartmap_startmode1$' --output-on-failure -j1
100% tests passed, 0 tests failed out of 1
Total Test time (real) =   3.96 sec

$ OMP_NUM_THREADS=1 ctest --test-dir build -R '^test_chartmap_aphi_abscissa$|^test_boozer_chartmap$|^test_chartmap_scaling$|^test_boozer_chartmap_roundtrip$|^test_chartmap_startmode1$' --output-on-failure -j1
100% tests passed, 0 tests failed out of 5
Total Test time (real) =   8.43 sec

$ OMP_NUM_THREADS=1 ctest --test-dir build -R '^test_chartmap_pipeline$|^test_chartmap_rz_consistency$|^test_coord_transform_roundtrip$' --output-on-failure -j1
100% tests passed, 0 tests failed out of 3
Total Test time (real) =  94.36 sec

$ OMP_NUM_THREADS=1 ctest --test-dir build -R '^test_e2e_boozer_chartmap$' --output-on-failure -j1
100% tests passed, 0 tests failed out of 1
Total Test time (real) = 102.43 sec

$ check-writing-slop.py test/tests/test_chartmap_startmode1.py
PASS: no writing-slop candidates at threshold medium

External W7X/GVEC reruns

Run with OMP_NUM_THREADS=1 using the rebased fix/chartmap-aphi-rho-abscissa build. These cases do not use generate_start_only, so they were not rerun after cf3ff9e.

W7X/vmec/simple-s1-p4k:
  final confined_fraction.dat line:
  1.0000000000000000E-003  0.67993164062500000  1.4648437500000000E-003  4096

W7X/vmec2gvec/simple-f1-n50/s316-p1k:
  final confined_fraction.dat line:
  9.9999999999999980E-004  0.64746093750000000  1.0742187500000000E-002  1024
  start.dat radial min=max=0.56213877290220782 = sqrt(0.316)

W7X/vmec2gvec/simple-f1-n50/s317-p1k:
  final confined_fraction.dat line:
  9.9999999999999980E-004  0.64843750000000000  4.8828125000000000E-003  1024
  start.dat radial min=max=0.5630275304103699 = sqrt(0.317)