YaochangLiu/narraiva-arc ? reverse-engineered prompt

Build me the first working version of this story generation engine.

I want a Python package that can take a simple sentence or a short creative brief and turn it into a structured long form story run, even when it is completely offline. It should have a clean public interface, a versioned story brief input format, deterministic results, checkpoint and resume support, and a safety check before final output is returned. Please make sure the offline provider can generate a small synthetic story path end to end, and that the result includes a final markdown artifact.

Keep the project boundaries clear, this is just the portable engine, not any hosted product or cloud stuff. Add the basic tests, examples, and command line entry points so I can run it locally and see it work. If anything is unclear, use the docs in the repo and look up current Python docs online if you need to.