Visitor: sync post code blocks to repo exactly (post used a completely different design - return-value visitors vs repo's void/print-in-place visitors, different Triangle model), split merged Circle/Rectangle/Triangle and AreaCalculator/PerimeterCalculator blocks, Java 25, H6 file labels, fix stale console output, README mapping

03-behavioral/visitor/README.md

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+# Visitor Design Pattern — Java Example
+
+**Pattern:** Behavioral → Visitor
+**Article:** https://ankurm.com/visitor-design-pattern-java/
+
+## What this example shows
+
+A shape hierarchy that never changes, with operations added as separate visitor classes instead of methods on each shape. `ShapeVisitor` declares one `visit()` overload per concrete type. `Shape` declares `accept(ShapeVisitor)` — the double-dispatch trick: the shape hands itself to the visitor as a concretely-typed `this`, so overload resolution picks the right `visit()` method with no `instanceof` checks anywhere. `Circle`, `Rectangle`, and `Triangle` are plain data holders that know nothing about area or perimeter. `AreaCalculator` and `PerimeterCalculator` are two concrete visitors — adding the second one required zero changes to any shape class. `Main` runs both visitors across the same three shapes.
+
+## How to run
+
+```bash
+javac visitor/*.java -d out/visitor
+java -cp out/visitor visitor.Main
+```
+
+Requires Java 25.
+
+## Post Section ↔ File Mapping
+
+| Post Section | File(s) |
+|---|---|
+| Implementation: Shape Geometry Operations — the Visitor interface | `ShapeVisitor.java` |
+| Implementation: Shape Geometry Operations — the Element interface | `Shape.java` |
+| Implementation: Shape Geometry Operations — Circle | `Circle.java` |
+| Implementation: Shape Geometry Operations — Rectangle | `Rectangle.java` |
+| Implementation: Shape Geometry Operations — Triangle | `Triangle.java` |
+| Implementation: Shape Geometry Operations — AreaCalculator | `AreaCalculator.java` |
+| Implementation: Shape Geometry Operations — PerimeterCalculator | `PerimeterCalculator.java` |
+| Implementation: Shape Geometry Operations — wiring it together | `Main.java` |
+
+Article: https://ankurm.com/visitor-design-pattern-java/
+All patterns: https://ankurm.com/design-patterns-java/