Drop legacy chartmap file support; fix chartmap example and README

[krystophny]

Follow-up to the review comments on #334 (cc @Rykath — this is now changed).

Risk tier

• T2: local numerical logic

Correctness contract

Intended behavior change

• read_boozer_chartmap no longer supports legacy chartmap files. It now requires:
  • the rmajor global attribute (previously optional via a has_rmajor flag that defaulted rmajor to 0.0), and
  • the endpoint-included theta_field/zeta_field Bmod grid (previously fell back to the geometry grid).
    GVEC-exported chartmaps (tools/gvec_to_boozer_chartmap.py) always write both, so current files are unaffected. Pre-converter legacy files now abort with a clear NetCDF error instead of being silently accepted with rmajor = 0.

Behavior that must not change

• map2disc-based chartmaps: unaffected. They are read through the libneo coordinate reader (make_chartmap_coordinate_system), not read_boozer_chartmap.
• Field values, scaling, and the symplectic/Boozer paths for valid current-format files.

Coordinate / unit conventions

• Unchanged. Bmod stays on the endpoint-included field grid spanning the full 2pi (poloidal) and 2pi/nfp (toroidal) period.

Numerical invariants

• Unchanged.

Tests added

• unit: updated generate_test_boozer_chartmap.c to emit the current format (rmajor + field-grid Bmod); updated test_chartmap_aphi_abscissa.f90 (the rmajor-present check is now covered by mandatory read + value assertion).

Golden-record impact

• unchanged

Failure modes considered

• Legacy file fed to the strict reader: now aborts with NetCDF error at att rmajor / inq_dim theta_field, by design.

Manual validation

Also addresses the two doc comments on #334:

• examples/simple_chartmap.in: dropped the forced startmode = 2 (the comment was incorrect, and it required a start.dat). The default startmode = 1 samples the sbeg flux surface and needs no start.dat.
• README.md: removed the incorrect "startmode = 2 means particle coordinates are in chartmap Boozer space" bullet (startmode is field-type independent).

Verification

Failing before (strict reader against the old legacy-format test generator):

read_boozer_chartmap: NetCDF error at att rmajor: NetCDF: Attribute not found
ERROR STOP read_boozer_chartmap failed
0% tests passed, 1 tests failed out of 1
	 20 - test_boozer_chartmap (Failed)

Passing after (generator updated to current format):

1/5 Test #18: test_chartmap_aphi_abscissa ......   Passed
2/5 Test #20: test_boozer_chartmap .............   Passed
3/5 Test #21: test_chartmap_startmode1 .........   Passed
4/5 Test #22: test_chartmap_scaling ............   Passed
5/5 Test #23: test_boozer_chartmap_roundtrip ...   Passed
100% tests passed, 0 tests failed out of 5

Pre-existing, unrelated to this PR: the *_map2disc chartmap tests fail on a clean main in this environment because the optional libneo Python binding _efit_to_boozer is not built (ModuleNotFoundError: No module named '_efit_to_boozer'). This PR introduces no new failures relative to that baseline.

[Rykath]

Looks good to me!