Executes an inline draft definition in a zero-side-effect simulator: expressions and transforms run for real, action.call steps are stubbed (fixture > schema-shaped placeholder > empty object) with their evaluated inputs recorded, and waits pause the run unless a fixture resumes them. Omit payload to have one synthesized from the trigger event schema or the workflow payload schema. Returns status, a per-step trace, final vars/payload, the stubbed action invocations, and any errors or warnings.