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.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+# Quick ad hoc JMH run against one benchmark class. Example:
+#   ./scripts/run.sh IncrementBenchmark -t 8
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+BENCH="${1:?Usage: run.sh <BenchmarkClassName> [extra jmh args]}"
+shift || true
+
+JAR=target/benchmarks.jar
+[ -f "$JAR" ] || { echo "Run 'mvn package' first."; exit 1; }
+
+java -jar "$JAR" "$BENCH" "$@"