Contract for rendering a WhatsApp pairing payload with a host-owned QR library.

ExWapp.pair/1 returns the raw payload; applications can render it using any library or send it to a browser renderer. ExWapp has no QR package dependency.

To use the manual ExWapp.Live runner, supply qr_renderer: MyApp.QRRenderer or set config :ex_wapp, :qr_renderer, MyApp.QRRenderer. Its :qr_options option is passed unchanged to render/2. The renderer returns SVG, PNG, or other file contents as binary/iodata; the runner writes them to :qr_path. Choose a path extension matching the format returned by the renderer.

For example, if your application chooses the qr_code package:

defmodule MyApp.QRRenderer do
  @behaviour ExWapp.QR.Renderer

  @impl true
  def render(payload, opts) do
    payload
    |> QRCode.create(Keyword.get(opts, :error_correction, :high))
    |> QRCode.render(Keyword.get(opts, :format, :svg))
  end
end

Return {:error, reason} when rendering fails. The runner returns that error and releases its QR subscription rather than waiting for an invisible code.