Async/concurrent/streaming discipline — five questions (overlapping runs, check-then-act, mutation across await, response ordering, cancellation), guard scope, fan-out failure, idempotent retries, shared-state audit, race amplification. Use for async fan-out (Promise.all, gather), streams/SSE/websockets, queue workers, cron jobs, webhooks, caches, locks, retries, and bugs described as "sometimes", "duplicate", "double-charged", or "only under load".
