Add Virtual Threads Tests, Fix Executor Shutdown Leak, and Optimize Predecessor Lookup

[rokon12]

Summary of Changes

• Virtual threads support (Java 21+): Added TaskPipelineVirtualThreadsTest covering 6 scenarios (compatibility, concurrency, fan-in, starvation, mixed workloads, thread-local safety) using reflection for Java 17 compatibility.
• Executor shutdown fix: Introduced timeout + awaitTermination (30s graceful, 10s forced) to prevent resource leaks; added logging and tests.
• Performance optimization: Replaced findPredecessorsFor O(n*m) lookup with reverse adjacency map for O(1) performance; added benchmarks and tests.

[sshaaf]

@rokon12 Thanks for contributing.