Function scan_frontier_cursor_cp 

pub fn scan_frontier_cursor_cp(
    checkpoint: Option<u64>,
    frontier: u64,
    direction: ScanDirection,
) -> Option<u64>

Resolve the checkpoint coordinate embedded in a transaction/event/checkpoint scan-frontier cursor independently from the optional completed-checkpoint claim. A missing mapping is representable only at the numeric edge where the frontier itself supplies the sole safe checkpoint coordinate.