Lesson 13: Refactoring to patterns

This lesson practices Refactoring to patterns at real C# application level. We start from design pain, study code, practice a small decision, predict behavior, and verify mastery.

Introduce patterns only after duplicated pressure appears.

Refactoring to patterns: Introduce patterns only after duplicated pressure appears.
design pressure: A repeated change, duplication, or test pain that signals the model needs a better boundary.