Add all 23 GoF design pattern implementations

03-behavioral/observer/Main.java

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+package observer;
+
+/**
+ * Observer Design Pattern — Runnable Demo
+ * Run: javac observer/*.java -d out/observer && java -cp out/observer observer.Main
+ * Article: https://ankurm.com/observer-design-pattern-java/
+ */
+public class Main {
+    public static void main(String[] args) {
+        System.out.println("=== Observer Design Pattern Demo ===\n");
+
+        StockMarket aapl = new StockMarket("AAPL", 175.00);
+
+        StockObserver alert    = new AlertObserver("RiskEngine", 2.0);
+        StockObserver alice    = new PortfolioObserver("Alice", 100);
+        StockObserver bob      = new PortfolioObserver("Bob", 50);
+
+        aapl.subscribe(alert);
+        aapl.subscribe(alice);
+        aapl.subscribe(bob);
+
+        System.out.println();
+        aapl.setPrice(178.50);  // +2% — should trigger alert
+
+        System.out.println();
+        aapl.setPrice(179.00);  // small move
+
+        System.out.println("\n-- Bob unsubscribes --");
+        aapl.unsubscribe(bob);
+
+        System.out.println();
+        aapl.setPrice(165.00);  // big drop — Bob doesn't hear it
+
+        System.out.println("\n=== Demo complete ===");
+    }
+}