boozer_sub: split chartmap file I/O into its own module

[krystophny]

Follow-up to #337. With the field-object graft gone, the only remaining non-converter dependency boozer_sub carried was the chartmap file I/O: export/load_boozer_from_chartmap pulled boozer_chartmap_io (the NetCDF reader) and netcdf into the module, so a VMEC→Boozer-only consumer had to compile them too.

What

Separate file-I/O from state-access (no submodule — plain modules, compiler-portable):

• boozer_chartmap_types (new): the boozer_chartmap_data_t record alone — no I/O, no deps beyond iso_fortran_env.
• boozer_sub keeps build_boozer_from_chartmap(d) — the spline-building half, which needs the private batch-spline state. Its only chartmap reference is now the trivial types module; it no longer uses boozer_chartmap_io or netcdf.
• boozer_chartmap (new) owns the file entry points: load_boozer_from_chartmap parses via boozer_chartmap_io then calls build_boozer_from_chartmap; export_boozer_chartmap gathers through the public splint_boozer_coord/boozer_to_vmec and writes NetCDF.

boozer_chartmap_io re-exports boozer_chartmap_data_t from the types module, so existing use boozer_chartmap_io, only: boozer_chartmap_data_t imports are unaffected. The only API change: callers of load/export_boozer_chartmap switch use boozer_sub → use boozer_chartmap (only test_boozer_chartmap_roundtrip in-tree; updated here).

Verification

test_boozer_chartmap_roundtrip   exit 0   (export+load via the new boozer_chartmap module)
test_boozer_converter_vs_simple  exit 0
RABE quick+slow:                 100% tests passed, 0 failed out of 32

Converter-only consumers (e.g. RABE's curated build) now compile boozer_converter with just boozer_chartmap_types — no boozer_chartmap_io, no boozer_chartmap, no NetCDF-via-chartmap.