Lightweight retry helper for strategies.

Tracks attempt counts inside the strategy's state map (under :__retries) so strategies can declaratively retry failed steps without hand-rolling counters.

Usage

alias Cyclium.Strategy.Retry

# On synthesis success — reset the counter
def handle_result(state, %{kind: :synthesis}, {:ok, result}) do
  {:ok, state |> Retry.reset(:synthesis) |> Map.put(:assessment, result)}
end

# On synthesis failure — retry up to 3 times with 2s backoff
def handle_result(state, %{kind: :synthesis} = step, {:error, _}) do
  case Retry.check(state, step, max_attempts: 3, backoff_ms: 2_000) do
    {:retry, new_state}             -> {:retry, new_state}
    {:give_up, _attempts, new_state} -> {:abort, "synthesis_failed_after_retries"}
  end
end

The strategy's next_step/2 doesn't need special logic — when handle_result returns {:retry, state}, the runner calls do_loop which calls next_step again. The strategy should naturally re-emit the same step type based on its phase/state (which hasn't changed, only :__retries was updated).