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

03-behavioral/visitor/Triangle.java

@@ -0,0 +1,10 @@
+package visitor;
+
+public class Triangle implements Shape {
+    private final double base, height;
+    public Triangle(double base, double height) { this.base = base; this.height = height; }
+    public double getBase()   { return base; }
+    public double getHeight() { return height; }
+    @Override public String getName() { return "Triangle(b=" + base + ",h=" + height + ")"; }
+    @Override public void accept(ShapeVisitor visitor) { visitor.visit(this); }
+}