Build a toy durable execution engine

The engine is one decorator and one function. @step wraps a function so each call takes the next sequence number in the run and looks it up in the journal: a completed entry is returned without executing, a missing one is executed and appended. run() sets up the run's context, calls the workflow and records the outcome. The decision that matters most is what a step does with each kind of exception: RetryableError is retried, FatalError is journaled and stops the run, and anything else is treated as a crash and journals nothing, so the step runs again on the next attempt.