Memento: sync post code blocks to repo exactly (add missing Editor methods), Java 25, H6 file labels, fix stale console output, README mapping

README.md

@@ -372,6 +372,23 @@ javac 03-behavioral/mediator/*.java -d out/mediator
 java -cp out/mediator mediator.Main
 ```
 
+### Memento (`03-behavioral/memento/`)
+
+| Post Section | File(s) |
+|---|---|
+| The Memento: Immutable and Opaque | `EditorMemento.java` |
+| The Originator (Editor) | `Editor.java` |
+| The Caretaker (History) | `History.java` |
+| Wiring it together (Main demo) | `Main.java` |
+
+Note: the "Serialization as Memento" snippet (`ObjectOutputStream`/`ObjectInputStream` example) is illustrative only — it is not part of this repository's runnable example.
+
+Run it:
+```bash
+javac 03-behavioral/memento/*.java -d out/memento
+java -cp out/memento memento.Main
+```
+
 ## Reference
 
 - *Design Patterns: Elements of Reusable Object-Oriented Software* — Gamma, Helm, Johnson, Vlissides