Pop revive service may process large revive offsets out of order

### Description

PopReviveService builds its checkpoint list with a comparator that subtracts two long revive offsets and casts the difference to int. Offsets separated by more than Integer.MAX_VALUE can be reversed, causing mergeAndRevive to process checkpoints in the wrong order.

### Expected behavior

Compare revive offsets with Long.compare so checkpoint order remains ascending across the full long range.

Start in PopReviveService where the checkpoint list comparator is built, then trace how mergeAndRevive consumes that list. Replace the offset comparison so values across the full long range remain in ascending order, and verify that large revive offsets are no longer processed out of order.

Written by the indexing model from the issue text.