Text-based stimulus: A four-step flowchart titled 'Modular Refactor Plan'. Boxes in order: Identify responsibilities, Extract one module, Define interface, Add boundary tests. It includes arrows left to right. It also includes a small label 'repeat' above an arrow curving from Add boundary tests back to Identify responsibilities. A team wants to move from a single file to modules while keeping the app running. Which migration plan is most modular?