Design a Workflow Engine

Problem Design a configurable workflow / approval engine using appropriate design patterns.

Requirements

• Define workflows as states, transitions, and actions.
• Provide a builder to construct a workflow and a factory for action handlers.
• Execute a workflow and persist its state across steps.

Areas to design

• The state-machine model (states, guarded transitions, actions on transition).
• Which design patterns earn their keep (Builder, Factory, State) and where they'd over-engineer.
• Extensibility for adding new action types without editing the engine core.