If you have ever built a modern fintech application, SaaS billing platform, or e-commerce engine, you already know a dirty little secret of software engineering: money does not move instantly. In a synchronous, closed-loop software architecture, state transitions are a walk in the park. When a user transfers funds within a single monolithic application, your database transaction commits instantly, and the glorious invariant that debits must equal credits is preserved atomically. But the real...