The step/handle outcomes, with shape validation.

{:next, step, state}        # transition, runnable, attempt := 0
{:next, step, state, opts}  # …with per-transition opts: rate_limit, weight
{:retry, state, delay_ms}   # same step, runnable, attempt += 1, eligible_at += delay
{:await, names, next_step, state}       # park; on any of `names`, run next_step
{:await, names, next_step, state, opts} # …with opts: timeout (ms; wake with what's
                                        # in the inbox when it fires)
{:done, result}             # terminal, done
{:stop, reason}             # terminal, failed

Step names and signal names are normalized to strings. :await accepts a single name or a list of names; both normalize to a list. :next accepts an optional 4th keyword opts (rate_limit:, weight:), :await an optional 5th (timeout:); both normalize to an opts map in the tuple's last position.