A payment is a claim. A receipt is a claim you can check. Four fields, one hash, a chain you can replay.
What this shows. Each receipt is hashed with the one before it, so any edit to an old entry breaks every entry after it. That is the whole argument: you do not have to trust the person who paid you, or the person who was paid, only the chain.
What it is not. A real deployment signs entries with a key per participant and keeps the log on a server none of the parties control. This is a single-file reference demo with local storage, no accounts and no server.