Demo: fan-out to three services with a deadline

Step by step

1. Create three simulated clients with delays 100, 150, and 400 ms using a dedicated ioPool.
2. Start all three, combine with thenCombine; measure total about 400 ms, not 650.
3. Make reviews fail; add exceptionally for an empty list and a degraded flag.
4. Add completeOnTimeout of 300 ms for reviews; show partial results.
5. Planned mistake: call .join() inside a thenApply on the common pool, then run many requests and watch throughput stall. Fix: compose futures (thenCombine/thenCompose) and join only at the end.
6. Print timings per scenario.