Enforce a new ordering invariant in writes to the log service: a Checkpoint-Completion-Marker in the Phylog should be physically located later in the cross-domain log than the oplog entries which are less than or equal to the checkpoint's stable oplog lsn. This is a stated assumption of the disagg storage design (see comments for justification) but is not currently enforced.

Add a guard to _pickNextProvider(): If the log server manager has a CHECKPOINT_END at the front of its queue, ensure that the log has written up to the stable oplog lsn captured by the checkpoint before appending the phylog entry. This will prevent the checkpoint reaching the log before the km entry.

Enforcing the new invariant will also prevent fatal crashes like BF-45689 (Full BF Write Up: https://docs.google.com/document/d/1cHtuRi-rBjn8hu6vETT9-c7G18FCRtt0gdxDMoahpQI/edit?usp=sharing).