atomics: Java Atomics and VarHandle companion code

AtomicLong/LongAdder/VarHandle counters benchmarked against the synchronized
and ReentrantLock baselines from the locks module, a deterministic ABA race
against a hand-rolled Treiber stack plus the AtomicStampedReference fix, and
a VarHandle access-modes demo (plain/opaque/acquire-release/volatile).

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

atomics/scripts/run-all.sh

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+# Regenerates every file in output/ using the same commands used to produce the ones
+# committed here. Requires JDK 25 on PATH/JAVA_HOME.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+mvn -q package
+
+echo "--- 01: IncrementBenchmark thread-count sweep (takes a few minutes) ---"
+{
+  echo '$ java -jar target/benchmarks.jar IncrementBenchmark -t <N> -rf text'
+  for t in 1 2 4 8 16 32 64; do
+    echo "=== threads=$t ==="
+    java -jar target/benchmarks.jar "IncrementBenchmark" -t "$t" -rf text -rff /tmp/jmh-atomics-$t.txt
+    cat "/tmp/jmh-atomics-$t.txt"
+    echo ""
+  done
+} > output/01-increment-benchmark-sweep.txt
+
+echo "--- 02: ABA problem demo ---"
+{
+  echo '$ java -cp target/classes com.ankurm.atomics.AbaProblemDemo'
+  echo ""
+} > output/02-aba-problem-demo.txt
+java -cp target/classes com.ankurm.atomics.AbaProblemDemo >> output/02-aba-problem-demo.txt 2>&1
+
+echo "--- 03: VarHandle access modes demo ---"
+{
+  echo '$ java -cp target/classes com.ankurm.atomics.VarHandleAccessModesDemo'
+  echo ""
+} > output/03-varhandle-access-modes.txt
+java -cp target/classes com.ankurm.atomics.VarHandleAccessModesDemo >> output/03-varhandle-access-modes.txt 2>&1
+
+echo "--- 04: correctness tests ---"
+mvn -q test
+cp target/surefire-reports/com.ankurm.atomics.CounterCorrectnessTest.txt output/04-counter-correctness.txt