A dictionary is read outside the lock. If the dictionaries folder changed
while it was being read (File › Options › Proofing), the reload emptied
the slot first and the late load then filled it with the old folder's
dictionary. It stayed until the next reload. Every language's copy of
the loader had this window; the shared slot now closes it in one place.
The slot counts reloads. A load that started before a reload still
answers the check that asked for it, but it isn't kept: the next check
reads the new folder.
Found by CodeRabbit on storytold/wordcraft#4191. Tests: a_load_from_before_a_reload_is_not_kept;
not counting reloads fails it. Proof: 288 tests.