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

03-behavioral/visitor/ShapeVisitor.java

@@ -0,0 +1,8 @@
+package visitor;
+
+/** Visitor interface — one visit() method per concrete element type */
+public interface ShapeVisitor {
+    void visit(Circle circle);
+    void visit(Rectangle rectangle);
+    void visit(Triangle triangle);
+}