Immutable conversation history for signature-shaped Imp programs.

A history is a sequence of prior task turns keyed by the same fields as the signature, not raw chat messages. For a signature like "question, history -> answer", a turn can be %{question: "Capital of France?", answer: "Paris"}. Turns are normalized through Imp.Example, so a turn must be a field map or a list of field pairs.

The Chat adapter renders each turn as prior user/assistant messages by splitting the fields according to the active signature, which keeps a history serializable and independent of any one provider's chat message schema.

iex> history =
...>   Imp.History.new()
...>   |> Imp.History.append(%{question: "Capital of France?", answer: "Paris"})
iex> Imp.History.messages(history)
[%{answer: "Paris", question: "Capital of France?"}]