th_fact made a ThingFact per placement, removal and use, and the valley places and removes Things
all day. The records come back two drains after they were handed out, so a reader placing a Thing
while it reads the last drain's list never sees one change (the test holds exactly that).
thing_use takes ThingsState mut, since it pushes a fact.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
