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/pom.xml

@@ -0,0 +1,60 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+
+  <!-- Same parent version as ../virtual-threads-benchmark and ../async: Boot 4.1.1 manages
+       Spring Framework 7.0.9 and Reactor 2024.x. Versions read from spring-boot-dependencies-4.1.1.pom,
+       not from release notes. Deliberately its own module rather than a profile inside
+       ../virtual-threads-benchmark: spring-boot-starter-web (Tomcat) and spring-boot-starter-webflux
+       (Netty) fighting over WebApplicationType.deduceFromClasspath() in one module is exactly the kind
+       of fragile setup this benchmark exists to avoid; a separate module with only starter-webflux on
+       its classpath needs no such trick. -->
+  <parent>
+    <groupId>org.springframework.boot</groupId>
+    <artifactId>spring-boot-starter-parent</artifactId>
+    <version>4.1.1</version>
+    <relativePath/>
+  </parent>
+
+  <groupId>com.ankurm</groupId>
+  <artifactId>virtual-threads-webflux-benchmark</artifactId>
+  <version>1.0.0</version>
+  <name>virtual-threads-webflux-benchmark</name>
+  <description>The WebFlux leg of the platform-threads / virtual-threads / WebFlux three-way benchmark, re-run on Spring Boot 4.1.1 / JDK 25</description>
+
+  <properties>
+    <java.version>25</java.version>
+  </properties>
+
+  <dependencies>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-webflux</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-actuator</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-test</artifactId>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
+      <groupId>io.projectreactor</groupId>
+      <artifactId>reactor-test</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.springframework.boot</groupId>
+        <artifactId>spring-boot-maven-plugin</artifactId>
+      </plugin>
+    </plugins>
+  </build>
+</project>