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

Fixes 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.
- Single-trial HTTP load tests on this shared sandbox swung by more than 50%
  run to run (795ms-1247ms observed on the identical /io endpoint back to
  back) -- large enough to flip which threading model looked faster. Fixed
  by taking the median of 5 independent trials for the I/O-bound benchmark
  and the median of 3 for the event-loop-starvation benchmark, rather than
  reporting a single noisy run as if it were precise.
- 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; raising to 60 fixed
  the under-loading problem but still flaked once during verification
  (372ms vs 374ms p99, a real tie). Final fix: 150 concurrent requests plus
  the median-of-3 trials above.

Also adds StreamBackpressureTest, a StepVerifier proof that the /stream
endpoint never emits ahead of its subscriber's outstanding requests, and
updates the module's docs to report the de-noised numbers with an explicit
methodology note on how they compare to the single-trial platform/virtual-
thread numbers reused from a different post.

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.