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

02-structural/adapter/PaymentGateway.java

@@ -0,0 +1,12 @@
+package adapter;
+
+/**
+ * The Target interface — what YOUR application's code expects.
+ * Every payment gateway in your system must implement this.
+ * Written to handle modern async-style payment flows.
+ */
+public interface PaymentGateway {
+    boolean charge(String customerId, double amount, String currency);
+    boolean refund(String transactionId, double amount);
+    String getStatus(String transactionId);
+}