Investigate adding the validation of the late-cutoff design (see https://docs.google.com/document/d/1ThhwS1BoVsxTQoXKM1NLJIY_Pkpb60h9qWv1-ZMX_zQ/edit?tab=t.r1zjyr7zhojz#heading=h.xyqckf2czhm6) to checkpoint resolution. Specifically, track the latest timestamp of phylog encryptors we mint, and ensure that the keystore we pick during checkpoint resolution is covered by this timestamp.