Function terminal_boundary_watermark 

pub fn terminal_boundary_watermark(
    options: &QueryOptions,
    end_checkpoint: u64,
    end_position: u64,
) -> Watermark

Boundary watermark emitted once a scan has drained its entire resolved range under natural completion. Unlike per-item watermarks it can claim the range’s final checkpoint complete — end_checkpoint - 1 ascending (the exclusive cp upper) or end_checkpoint descending (the inclusive cp lower) — because no further items exist in it within the requested range. The (end_checkpoint, end_position) cursor resumes exactly past the scanned range.