LLD Lab Stage 3: Set L3 · The everyday six patterns

7 sections: Strategy, Factory (+ Factory Method), Builder, Singleton
(+ sins + double-checked locking + the module-level free-singleton
insight), Observer, and Decorator, each as violation-code -> pattern
refactor with a real-life analogy and diagram. Cross-references L1's
engine slot / Notifier / composition-explosion and L2's YAGNI / DIP
throughout, and every section links back to its line in the existing
signal router. Adds 5 figures, 6 ladder drills, 6 flashcards, 6 journal
questions. Atlas L3 row and sidebar chip updated to L0-L3 live, L4 next.

Author: DeepCodeWork