feat: implement Immutable pattern (Fixes #3448) (#3509)

* feat: implement Immutable pattern #3448

Add immutable/ module with ImmutableUser (final class, defensive copy,
withAge wither method), App demo, JUnit 5 tests, PlantUML diagram, and
README following project conventions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* feat: add withName and withRoles wither methods to ImmutableUser #3448

Every field now has a corresponding wither for consistency.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix: add junit-jupiter-api dependency to immutable module #3448

Required for @Test annotations and assert methods in test code.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

immutable/etc/immutable.urm.puml

@@ -0,0 +1,21 @@
+@startuml
+package com.iluwatar.immutable {
+  class App {
+    - LOGGER : Logger {static}
+    + App()
+    + main(args : String[]) {static}
+  }
+  class ImmutableUser {
+    - name : String {final}
+    - age : int {final}
+    - roles : List<String> {final}
+    + ImmutableUser(name : String, age : int, roles : List<String>)
+    + getName() : String
+    + getAge() : int
+    + getRoles() : List<String>
+    + withAge(newAge : int) : ImmutableUser
+    + toString() : String
+  }
+}
+App ..> ImmutableUser : creates
+@enduml