executors: ExecutorService lifecycle companion code

Adds the executors module: shutdown() vs shutdownNow(), the graceful
two-phase shutdown pattern, AutoCloseable/try-with-resources (JDK 19+),
newVirtualThreadPerTaskExecutor() timing vs a bounded pool, and the
submit()-without-get() lost-exception trap. 5 runnable demos, 1 JUnit
correctness test class (23 tests), 6 captured output transcripts.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01FhzLY5p6okFva3qsnsRyvM

executors/output/01-shutdown-vs-shutdownnow.txt

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+=== shutdown(): lets running AND already-queued tasks finish ===
+task-0: started
+task-1: started
+main: called shutdown() - isShutdown=true, isTerminated=false
+task-0: finished normally
+task-2: started
+task-1: finished normally
+task-3: started
+task-3: finished normally
+task-4: started
+task-2: finished normally
+task-5: started
+task-4: finished normally
+task-5: finished normally
+main: awaitTermination returned true - all 6 tasks ran to completion, none were skipped
+
+=== shutdownNow(): interrupts running tasks, abandons queued ones ===
+task-0: started
+task-1: started
+main: called shutdownNow() - 4 queued tasks abandoned, never started
+task-1: interrupted, exiting early
+task-0: interrupted, exiting early
+main: awaitTermination returned true - the 2 running tasks were interrupted mid-sleep, not allowed to finish
+done

executors/output/02-graceful-shutdown-pattern.txt

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+=== Case 1: tasks finish comfortably inside the grace period ===
+quickPool: shutdown() called, waiting up to 1s for a clean finish
+quick-task-1: done (200ms)
+quick-task-0: done (200ms)
+quickPool: terminated cleanly within the grace period
+
+=== Case 2: a task outlives the grace period, forcing shutdownNow() ===
+slowPool: shutdown() called, waiting up to 1s for a clean finish
+slow-task: started, will try to sleep 3s
+slowPool: grace period expired, escalating to shutdownNow()
+slow-task: interrupted during shutdownNow(), stopping early
+slowPool: terminated after shutdownNow()
+done

executors/output/03-autocloseable-executor.txt

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+=== try-with-resources: close() runs the graceful pattern automatically ===
+main: leaving the try-with-resources block now
+task-2: finished
+task-0: finished
+task-1: finished
+main: back from the try-with-resources block - the pool is terminated, every task already finished, no separate awaitTermination() call was written
+done

executors/output/04-virtual-thread-per-task-executor.txt

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+Running 5000 tasks that each sleep 50ms.
+
+sample task thread: VirtualThread[#22]/runnable@ForkJoinPool-1-worker-1
+newVirtualThreadPerTaskExecutor(): 5000/5000 tasks completed in 121ms
+(121ms is close to the 50ms a single task takes - every task ran concurrently, nothing waited for a free worker)
+
+sample task thread: Thread[#5027,pool-1-thread-1,5,main]
+newFixedThreadPool(20): 5000/5000 tasks completed in 12556ms
+(12556ms is roughly 250 rounds of 50ms each - only 20 tasks are ever running at once)
+done

executors/output/05-lost-exception-trap.txt

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+=== execute(): uncaught exception reaches the default handler, gets printed ===
+Exception in thread "pool-1-thread-1" java.lang.RuntimeException: boom from execute()
+	at com.ankurm.executors.LostExceptionTrapDemo.lambda$main$0(LostExceptionTrapDemo.java:22)
+	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1090)
+	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:614)
+	at java.base/java.lang.Thread.run(Thread.java:1474)
+
+=== submit(): same exception, but nobody calls get() - dead silence ===
+main: task's Future exists (isDone=true), but its exception was never printed anywhere - it's just sitting inside the Future
+
+=== submit() + get(): the SAME exception, now surfaced by calling get() ===
+main: caught ExecutionException, cause = java.lang.RuntimeException: boom from submit(), retrieved via get()
+done

executors/output/06-correctness-tests.txt

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+-------------------------------------------------------------------------------
+Test set: com.ankurm.executors.ExecutorShutdownBehaviorTest
+-------------------------------------------------------------------------------
+Tests run: 23, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.417 s -- in com.ankurm.executors.ExecutorShutdownBehaviorTest