Function boundary_cursor_cp 

pub fn boundary_cursor_cp(cp: u64, direction: ScanDirection) -> u64

Resolve the boundary-cursor checkpoint coordinate for a list_transactions / list_events scan frontier. The cursor encoding is asymmetric: ascending Boundary cursors advance the cp-range start, so the frontier cp is used directly; descending Boundary cursors treat the cp coordinate as an EXCLUSIVE upper bound, so cp + 1 is needed to keep cp itself included on resume.