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

03-behavioral/state/RedState.java

@@ -0,0 +1,11 @@
+package state;
+
+public class RedState implements TrafficLightState {
+    @Override public void onEnter(TrafficLight light) {
+        System.out.println("  [RED]   Stop. Pedestrians cross.");
+    }
+    @Override public void next(TrafficLight light) {
+        light.setState(new GreenState());
+    }
+    @Override public String getColor() { return "RED"; }
+}