Behaviour for pluggable payment method verification.

A payment method connects the MPP protocol to a real payment system (Stripe, Tempo, x402/EVM, etc.). Each method module implements three callbacks that the MPP.Plug middleware uses during the 402 handshake:

1. method_name/0 — lowercase identifier for protocol headers
2. verify/2 — verify a credential payload against a charge intent
3. challenge_method_details/1 — (optional) add method-specific fields to challenges

Usage

defmodule MyApp.Payments.Stripe do
  use MPP.Method

  @impl MPP.Method
  def method_name, do: "stripe"

  @impl MPP.Method
  def verify(payload, charge) do
    # payload is the credential's method-specific proof map
    # charge is the validated MPP.Intents.Charge struct
    spt = Map.fetch!(payload, "spt")
    # ... verify with Stripe API ...
    {:ok, MPP.Receipt.new(method: "stripe", reference: "pi_abc123")}
  end

  @impl MPP.Method
  def challenge_method_details(_charge) do
    %{"networkId" => "...", "paymentMethodTypes" => ["card"]}
  end
end

Design

Intent = Schema, Method = Implementation. All methods share the same MPP.Intents.Charge struct — methods only handle verification. The payload argument is the raw map from the credential (method-specific proof), not the full MPP.Credential struct. Methods don't need to know about protocol framing.