chemx-ext — interfaces to external programs (xtb, CREST) and the
RDKit-free fallback conformer generator.
The subprocess wrappers never require the external binary at build or test
time: the input writers and output parsers are pure functions tested against
vendored fixture strings (see PROVENANCE.md for the format sources), and
binary detection ([xtb::find_xtb], [crest::find_crest]) returns a named
[ExtError::BinaryNotFound] with install guidance when the program is
absent.