Find the latest known RisingWave commit boundary. A replace snapshot never changes table
contents, so its boundary is always its parent’s. Any marker on it is ignored: compaction copies
the marker of the snapshot it planned against, but commits on top of the latest head, so a sink
commit that lands in between makes the copied marker stale. We deliberately stop at any other
unmarked operation: an external or legacy append cannot be assigned a safe log-store boundary.