This ticket handles step-up: on a step-up signal, the applier drains and signals drain-complete so the replication coordinator can finish transitioning to primary.

This ticket should include:

• On the step-up signal, stop pulling and perform a full drain, then signal drain-complete to the replication coordinator.
• A drained pipeline implying no in-flight batches and no outstanding deferred writes, so the new primary starts from a fully-applied, consistent state.