Fix three issues from PR review

evenCheckpoints panic on tiny --target-max-pack-bytes: when numBatches >= len(chain), batchSize truncated to 0 and the index formula produced -1. Guard returns just the tip in this case.

Remove stale Mode field from ExecutionSummary: the rename to TransferMode left the old Mode field, causing both "mode" and "transfer_mode" to appear in JSON output. Only TransferMode remains.

Make replicate's closeOnceReadCloser thread-safe: switch from plain bool to sync.Once, matching the bootstrap package's implementation.