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

03-behavioral/visitor/Rectangle.java

@@ -0,0 +1,10 @@
+package visitor;
+
+public class Rectangle implements Shape {
+    private final double width, height;
+    public Rectangle(double width, double height) { this.width = width; this.height = height; }
+    public double getWidth()  { return width; }
+    public double getHeight() { return height; }
+    @Override public String getName() { return "Rectangle(" + width + "x" + height + ")"; }
+    @Override public void accept(ShapeVisitor visitor) { visitor.visit(this); }
+}