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

02-structural/proxy/DatabaseConnection.java

@@ -0,0 +1,11 @@
+package proxy;
+
+/**
+ * Subject interface — defines what both the real object and proxy expose.
+ * Clients depend on this, not on the concrete class.
+ */
+public interface DatabaseConnection {
+    void connect();
+    String executeQuery(String sql);
+    void disconnect();
+}