concurrency-interview: companion code for the Top 40 interview post

Adds the concurrency-interview module: a real race condition with its
actual lost-update count, why volatile alone doesn't fix it, a real
JVM-detected deadlock (ThreadMXBean.findDeadlockedThreads()) with the
fix, and ScopedValue's exact child-thread inheritance rules (finalized
in JDK 25 via JEP 506, demonstrated against StructuredTaskScope, still
preview per JEP 505/525). 4 runnable demos, 1 JUnit test class (9
tests), 5 captured output transcripts.

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

concurrency-interview/output/01-race-condition.txt

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+expected increments: 1600000
+unsafeCounter (plain int, unsynchronized): 1248496
+safeCounter (AtomicInteger): 1600000
+unsafeCounter lost 351504 updates to the race
+safeCounter matches expected: true

concurrency-interview/output/02-volatile-not-enough.txt

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+expected increments: 1600000
+volatileCounter: 855018 (volatile guarantees every thread SEES the latest value - it does not make ++ atomic)
+atomicCounter: 1600000
+volatileCounter lost updates: 744982
+atomicCounter matches expected: true

concurrency-interview/output/03-deadlock-detection.txt

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+=== Opposite lock order: a real deadlock, detected via ThreadMXBean ===
+thread-1: holding lockA, waiting for lockB
+thread-2: holding lockB, waiting for lockA
+main: findDeadlockedThreads() detected 2 deadlocked threads
+  thread-1 is blocked on java.lang.Object@5451c3a8, owned by thread-2
+  thread-2 is blocked on java.lang.Object@49476842, owned by thread-1
+
+=== Same lock order on both threads: no deadlock possible ===
+thread-1 (fixed order): acquired both locks, done
+thread-2 (fixed order): acquired both locks, done
+main: both fixed-order threads finished, isAlive t1=false t2=false
+done (the process exits here - the two deadlocked threads above never finish)

concurrency-interview/output/04-scoped-value-inheritance.txt

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+=== Same-thread binding: visible to direct AND indirect callees ===
+handleRequest(): REQUEST_ID.get() = req-42
+logSomewhereDeeper() (indirect callee, same thread): REQUEST_ID.get() = req-42
+
+=== Outside any binding: isBound() is false ===
+main: REQUEST_ID.isBound() = false
+
+=== A plain `new Thread()` does NOT inherit the binding ===
+plain child thread: REQUEST_ID.isBound() = false
+
+=== A StructuredTaskScope.fork()'d thread DOES inherit the binding ===
+forked subtask: REQUEST_ID.isBound() = true, REQUEST_ID.get() = req-100
+done

concurrency-interview/output/05-correctness-tests.txt

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+-------------------------------------------------------------------------------
+Test set: com.ankurm.concurrencyinterview.InterviewClaimsTest
+-------------------------------------------------------------------------------
+Tests run: 9, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.216 s -- in com.ankurm.concurrencyinterview.InterviewClaimsTest