Add multithreading module: odd-even, ping-pong, ordered threads, deadlock detection and fixes, virtual-thread variants (25 tests)

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_017ic4b7rnC7jppGGdHGESvy

multithreading/scripts/run-all.sh

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+#!/usr/bin/env bash
+# Regenerates every transcript in output/. Needs JDK 25 and Maven 3.9+.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+mkdir -p output
+./scripts/run.sh odd-even  > output/01-odd-even.txt
+./scripts/run.sh ping-pong > output/02-ping-pong.txt
+./scripts/run.sh ordered   > output/03-ordered-threads.txt
+./scripts/run.sh deadlock  > output/04-deadlock.txt
+./scripts/run.sh virtual   > output/05-virtual-single-carrier.txt
+mvn -B -q test > /dev/null 2>&1
+python3 - <<'PY' > output/06-test-results.txt
+import glob, xml.etree.ElementTree as ET
+total = fail = 0
+for f in sorted(glob.glob("target/surefire-reports/TEST-*.xml")):
+    for tc in ET.parse(f).getroot().iter("testcase"):
+        bad = tc.find("failure") is not None or tc.find("error") is not None
+        total += 1; fail += bad
+        print(("FAIL " if bad else "PASS ") + tc.get("classname").split(".")[-1] + "." + tc.get("name"))
+print(f"{total} tests, {fail} failures")
+PY

multithreading/scripts/run.sh

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+#!/usr/bin/env bash
+# Runs one demo: ./scripts/run.sh odd-even | ping-pong | ordered | deadlock | virtual
+# Needs JDK 25 and Maven (Maven only to compile). The "virtual" demo is started with ONE carrier thread.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+mvn -B -q compile 2>&1 | grep -E 'ERROR' || true
+FLAGS=(-Xmx256m)
+if [ "${1:-}" = "virtual" ]; then
+  FLAGS+=(-Djdk.virtualThreadScheduler.parallelism=1 -Djdk.virtualThreadScheduler.maxPoolSize=1)
+fi
+java "${FLAGS[@]}" -cp target/classes com.ankurm.interviewlab.multithreading.Demo "$1" 2>&1 | grep -v '^Picked up JAVA_TOOL_OPTIONS'