HCS-27: Publish a Checkpoint

This example publishes an HCS-27 checkpoint message.

What it does

Creates an HCS-27 checkpoint topic when HCS27_CHECKPOINT_TOPIC_ID is not set.
Publishes an inline HCS-27 checkpoint to the topic.
Publishes an overflow checkpoint that stores metadata via an hcs://1/... reference.
Fetches the topic records, prints the HRL reference, and validates the checkpoint chain.

export HEDERA_ACCOUNT_ID="0.0.xxxxx"
export HEDERA_PRIVATE_KEY="302..."
go run ./examples/hcs27-publish-checkpoint