Turns a permission stage into a price transform.

FixAlchemy.Feed keys its tree on stages and never interprets them. A venue implements this behaviour to say what each stage does to a price.

defmodule MyVenue.Prices do
  @behaviour FixAlchemy.Feed.Pipeline

  @impl true
  def apply_stage({:spread, points}, price) do
    %{price | bid: price.bid - points, ask: price.ask + points}
  end

  def apply_stage({:markup, points}, price) do
    %{price | ask: price.ask + points}
  end

  def apply_stage({:commission, :none}, price), do: price
end

apply_stage/2 is called once per live edge of the tree, not once per subscriber, and runs in the publishing process. It must be pure and must not block.