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/scripts/run-all.sh

@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+# Regenerates every file in ../output/. Requires JDK25_HOME (and JAVA_HOME pointed at it for the
+# Maven build). All demos in this module run on a single modern JDK - there's no cross-version
+# comparison needed here, unlike vt-pinning or synchronizers.
+set -euo pipefail
+
+if [[ -z "${JDK25_HOME:-}" ]]; then
+  echo "JDK25_HOME must be set (e.g. /path/to/jdk-25)" >&2
+  exit 1
+fi
+
+cd "$(dirname "$0")/.."
+OUT=output
+mkdir -p "$OUT"
+
+JAVA_HOME="$JDK25_HOME" mvn -q -f ../pom.xml -pl executors -am package
+
+run() {
+  local class=$1
+  local outfile=$2
+  echo "==> $class"
+  "$JDK25_HOME/bin/java" -cp target/classes "com.ankurm.executors.$class" 2>&1 | grep -v "Picked up" > "$OUT/$outfile"
+}
+
+run ShutdownVsShutdownNowDemo          01-shutdown-vs-shutdownnow.txt
+run GracefulShutdownPatternDemo        02-graceful-shutdown-pattern.txt
+run AutoCloseableExecutorDemo          03-autocloseable-executor.txt
+run VirtualThreadPerTaskExecutorDemo   04-virtual-thread-per-task-executor.txt
+run LostExceptionTrapDemo              05-lost-exception-trap.txt
+
+cp target/surefire-reports/com.ankurm.executors.ExecutorShutdownBehaviorTest.txt "$OUT/06-correctness-tests.txt"
+
+echo "Done. See $OUT/"