p13 prereg (7 Nov 02:40): do the p4 changepoints sit at logbook volume (folder_ref) boundaries?
Expect: if steps come from a new log volume (new copyist, new correction habit), cp within 10 days of a folder_ref change for >=7/13 drifters, vs <=3/13 for random dates drawn within each ship's record (null by 200 shuffles).
My guess: miss (I expect ~3/13), since volumes probably span whole years.
