TDD, one test cycle per task, green suite at every boundary. Task 4 carries the content swap and the generated-tree check (the Duncarrow lesson). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01QYa9u7Kdxv5gX4AnwWexy8