Add all 23 GoF design pattern implementations (2026-06-13)

03-behavioral/state/TrafficLightState.java

@@ -0,0 +1,8 @@
+package state;
+
+/** State interface — each concrete state handles events differently */
+public interface TrafficLightState {
+    void onEnter(TrafficLight light);
+    void next(TrafficLight light);
+    String getColor();
+}