Function build_checkpoint_with_chain_frontier 

pub fn build_checkpoint_with_chain_frontier(
    checkpoint_seq: u64,
    batch_start_seq: u64,
    batch_end_seq: u64,
    receipt_canonical_bytes_batch: &[Vec<u8>],
    keypair: &Keypair,
    previous_checkpoint: Option<&KernelCheckpoint>,
    prior_chain: &CheckpointChainFrontier,
) -> Result<KernelCheckpoint, CheckpointError>

Build a signed kernel checkpoint from the chain frontier rather than from every prior leaf.

This is the hot path: a long-lived writer keeps the frontier and extends it, so issuing a checkpoint costs O(log n) hashes instead of rehashing the whole chain. The predecessor’s signed chain_root is still checked, at the same O(log n) cost, so the integrity guarantee is unchanged.