LLD Lab Stage 2: Set L2 · Good design vocabulary + SOLID

8 sections: coupling/cohesion, DRY/KISS/YAGNI, then S/O/L/I/D each as a
violation-code -> refactor-code pair with its own real-life analogy and
diagram. Cross-references L1's payment/Bird examples throughout. Adds 5
figures, 5 ladder drills, 6 flashcards, 6 journal questions. Atlas L2 row
and sidebar chip updated to reflect L0-L2 live, L3 next.

Author: DeepCodeWork