The store now carries an inbuxa store version (1), kept in the fork's
subspace beside the 0.16 schema version, which stays as it is. A store
without the mark, from an older release or from Stalwart, gets it on
start. A store marked newer than the running release is refused before
anything writes to it, with a message pointing at the newer release or
the pre-upgrade backup.
Layout 1 is what 2026.x and 27.1 both write today, so the same check goes
to release/2026: a 2026.x release then refuses a store a later 27.x has
changed instead of opening it.
(cherry picked from commit f7ec11c107)