Add virtual-threads-benchmark-webflux: the WebFlux leg of the three-way benchmark

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.

virtual-threads-benchmark-webflux/docs/output/01-io-bound-webflux.txt

@@ -0,0 +1,8 @@
+I/O-bound endpoint (/io, Mono.delay(300ms)), concurrency=600, 2 vCPU sandbox, Spring Boot 4.1.1 / JDK 25.0.4.1
+==============================================================================================================
+webflux (netty) : total=600 success=600 wall=852ms p50=591ms p99=738ms
+
+Mono.delay() never parks a thread -- the event loop schedules a timer callback and
+goes back to the selector loop immediately. Wall time for 600 concurrent requests
+stays close to a single 300ms round, same shape as virtual threads' /io result in
+../virtual-threads-benchmark/docs/output/01-io-bound-benchmark.txt.

virtual-threads-benchmark-webflux/docs/output/02-cpu-bound-webflux.txt

@@ -0,0 +1,17 @@
+CPU-bound endpoint (/cpu vs /cpu-offloaded, 20,000x SHA-256), concurrency=60, 2 vCPU sandbox, Spring Boot 4.1.1 / JDK 25.0.4.1
+==============================================================================================================================
+LoopResources.DEFAULT_IO_WORKER_COUNT (Netty event-loop threads) = 4
+Runtime.availableProcessors() (Schedulers.parallel() thread count) = 2
+
+webflux naive (Mono.fromCallable, no subscribeOn)      : total=60 success=60 wall=293ms p50=168ms p99=271ms
+webflux offloaded (subscribeOn(Schedulers.parallel())) : total=60 success=60 wall=316ms p50=179ms p99=314ms
+
+Counter-intuitive result, and worth stating honestly rather than forcing the expected
+story: on THIS box, the two wall times are close, because Reactor Netty's default event-
+loop pool (DEFAULT_IO_WORKER_COUNT = max(availableProcessors(), 4) = 4 here) is actually
+LARGER than Schedulers.parallel()'s pool (sized to availableProcessors() = 2). The naive
+endpoint that "incorrectly" runs on the event loop has more worker threads to run on,
+at this modest concurrency, than the "correctly offloaded" one. This does not mean the
+naive version is fine -- see the event-loop-starvation scenario below for what it actually
+breaks -- only that per-endpoint throughput alone does not show the problem on a small,
+under-loaded box like this one.

virtual-threads-benchmark-webflux/docs/output/03-event-loop-starvation.txt

@@ -0,0 +1,16 @@
+/io latency while 60 concurrent CPU-bound requests run, 2 vCPU sandbox, Spring Boot 4.1.1 / JDK 25.0.4.1
+========================================================================================================
+/io alone (baseline, no concurrent CPU load)              : total=20 success=20 wall=336ms p50=323ms p99=329ms
+/io while 60x /cpu (naive) run concurrently          : total=20 success=20 wall=486ms p50=452ms p99=467ms
+/io while 60x /cpu-offloaded run concurrently        : total=20 success=20 wall=398ms p50=377ms p99=387ms
+
+This is the real cost of the naive endpoint, and it does not show up by benchmarking
+/cpu in isolation: /io shares the same small event-loop pool with /cpu. With enough
+concurrent CPU load to occupy all 4 event-loop threads for the full measurement
+window (a first cut of this test used only 8 concurrent CPU requests, which drained
+through the event loop in well under the /io window and produced an inconsistent,
+sometimes-inverted result across runs -- fixed by raising the CPU load's own
+concurrency, not by changing the assertion), naive /io latency is consistently and
+substantially worse than both the undisturbed baseline and the offloaded case. At
+higher production concurrency this is the exact mechanism behind a single CPU-heavy
+endpoint silently degrading every other endpoint on the same Netty server.

virtual-threads-benchmark-webflux/docs/output/04-stream-backpressure.txt

@@ -0,0 +1,13 @@
+Flux backpressure proof: /stream, requested in batches of 3, 2, then 45
+=======================================================================
+StepVerifier.create(controller.streamResults(), 3)
+  .expectNext("event-0", "event-1", "event-2")
+  .expectNoEvent(Duration.ofMillis(80))   -- no 4th item arrives without a request
+  .thenRequest(2).expectNext("event-3", "event-4")
+  .thenRequest(45).expectNextCount(45)
+  .expectComplete()
+
+RESULT: verified -- the flux emitted exactly as many items as were requested, in
+the order requested, with no items arriving ahead of a pending request. This is
+what "backpressure is part of the Flux contract" means concretely: the subscriber,
+not the producer, controls the emission rate.