Function boundary_watermark 

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

Build a standalone scan-frontier Watermark. cursor_cp / position are the boundary cursor coordinates the caller has already resolved for its scan domain (see boundary_cursor_cp for the per-checkpoint scanners’ direction adjustment); boundary is the accumulated completion boundary.