Use when writing or reviewing concurrent code that mutates shared state —
endpoints, workers, threads, goroutines, coroutines, async/await — or when
building tests that fire simultaneous operations and assert invariants. Covers
double-submit, TOCTOU, lost update, check-then-act, data race, deadlock, and
idempotency across many languages (Python, JS/TS, Java/Kotlin, C#, Go, Rust,
C/C++, Ruby, PHP, Swift, Dart) with per-language guards, examples, and lint
detectors. Triggers: "race condition", "concurrency", "data race", "double
submit", "lost update", "TOCTOU", "deadlock", "thread safety", "idempotency",
"lock", "mutex", "atomic".
