Fixes two bugs found during self-correction before publishing:
- /stream used Flux.interval(), which ticks on its own wall-clock schedule
independent of downstream demand and threw OverflowException under a slow
subscriber; switched to Flux.range(), which has no independent production
schedule and can never outrun demand.
- The event-loop-starvation test's first cut used only 8 concurrent /cpu
requests as background load, which drained through the 4 event-loop
threads well inside the /io measurement window and produced an
inconsistent, sometimes-inverted result across runs; raised to 60
concurrent requests (matching the CPU-bound benchmark's own concurrency)
so the load actually occupies the event loop for the full window.
Also adds StreamBackpressureTest, a StepVerifier proof that the /stream
endpoint never emits ahead of its subscriber's outstanding requests.
