Function advance_boundary_excluding_cp 

pub fn advance_boundary_excluding_cp(
    prev: Option<u64>,
    cp: u64,
    options: &QueryOptions,
) -> Option<u64>

Fold a cp whose own checkpoint is NOT proven complete into the accumulated boundary (list_transactions / list_events: cp C may still hold further matches at other tx_seqs / event_seqs; and any scan frontier, which lands partway through the checkpoint it resolves to). The boundary excludes C itself: C - 1 ascending / C + 1 descending.

When that adjusted candidate would overflow (C == 0 ascending or u64::MAX descending) the previously accumulated boundary is preserved rather than collapsed back to None.