core_version is a string the code states, null or keyed by build mode,
and the schema refuses Git, SVN, unknown, master, main, latest, dev and
HEAD, alone or annotated. version_basis is defined per kind with the
fields each one needs, and kind none requires a null version.
tests/test_version_basis.py reads a recorded fixture of forge answers
offline and fails on drift, holds every profile to a basis (tag versions
in describe form, aliases equal to their profile, none as null), checks
that a pin advance reads the version again or is refused, and that the
schema refuses placeholders.
