Function item_watermark 

pub fn item_watermark(
    options: &QueryOptions,
    cp: u64,
    position: u64,
    boundary: Option<u64>,
) -> Watermark

Build the embedded Watermark for an item: the cursor encodes this item’s position (so the next request’s after/before resumes past it) plus the current direction-matching checkpoint boundary. cp / position are the item’s cursor coordinates (list_checkpoints passes its cp_seq for both).