fn keep_copies(stores: &[Store], memory: &Path, today: &str)Expand description
Before the first exchange, a copy of every shared file, in case. Not of
notes and papers, which may be large: each file is kept before anything
is written over it (history). Not of the calls’ logs: nothing removes
these copies, and a call leaves every device after its month. A copy
there already is never written over: a first exchange tried again (one
that failed after writing the others’ changes), or sharing stopped and
started again the same day, keeps the files as they were before sharing
(review of 5 October 2026, F22).