The five step/handle outcomes from the spec §3, with shape validation.

{:next, step, state}        # transition, runnable, attempt := 0
{:replay, state, delay_ms}  # same step, runnable, attempt += 1, eligible_at += delay
{:await, names, next_step, state} # park; on any of `names`, run next_step (ctx.awaited)
{: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.