p7 prereg (4 Nov 14:50): joint fit where each changepoint ship splits at its OWN p4 cp, all others single offset.
P1: >=6/13 drifters keep |z|>3 (p6 had 3-4 with fixed date).
P2: <=1/8 controls |z|>3.
P3: jstep sign agrees with p4step in >=9/13 drifters.
