[Janaka2]

Motivation

• Provide side-by-side implementations showing Classic OO, functional/lambda-first, and Java 21+ idiomatic variants for common patterns to help compare styles.
• Demonstrate modern concurrency patterns (virtual-thread friendly producer/consumer, bounded backpressure queue, and thread confinement with immutable handoff) for Java 21+ audiences.
• Ship reference documentation and tests so the variants are discoverable and validated by CI-style checks.

Description

• Add a new docs page docs/phase-3-modern-java-variants.md describing the Phase 3 goals and examples.
• Add package org.example.modern_java with StrategyVariants, ObserverVariants, and CommandVariants showing Classic, Functional, and Modern (sealed/record/switch) implementations.
• Add ConcurrencyPatternsPack demonstrating producerConsumerWithVirtualThreads, BoundedBackpressureQueue<T> and ThreadConfinedMailbox<T> plus an ImmutableMessage record, and make ThreadConfinedMailbox use a dedicated ExecutorService and call shutdownNow() on close.
• Update README.md to include Phase 3 and the new modern_java package, and add unit tests in src/test/java/example/ModernJavaVariantsTest.java covering the new variants.

Testing

• Added ModernJavaVariantsTest which exercises the Strategy, Command, Observer, and Concurrency patterns; these tests are included in the repository as src/test/java/example/ModernJavaVariantsTest.java.
• Attempted to run mvn -Dtest=ModernJavaVariantsTest test, which failed in this environment because Maven could not resolve plugin dependencies from Maven Central (HTTP 403), so the new tests could not be executed here.

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Codex Task