Add virtual-threads-benchmark: re-run Spring Boot 4.1 / JDK 25 benchmarks, JEP 491 pinning fixed

Companion module for the rewritten post 'Virtual Threads on Spring Boot 4.1: The Benchmarks,
Re-Run, and the Pinning Advice That Expired', retitled and re-benchmarked on Boot 4.1.1 /
JDK 25.0.4.1 (the original post was written against Boot 3.4 / JDK 21). Covers: I/O-bound and
CPU-bound throughput (platform vs virtual threads, including a JIT-warmup benchmarking bug this
build caught and fixed), JEP 491 proof that synchronized no longer pins a virtual thread's
carrier across a blocking call as of JDK 24 (obsoleting the old avoid-synchronized advice),
proof that -Djdk.tracePinnedThreads=full is inert on JDK 25, and JEP 506's finalized ScopedValue
API (JDK 25 GA, no --enable-preview, and a different shape than the old preview API). Kept as
its own module rather than a new top-level repository, alongside the existing async/ module,
which already has a stronger dual-JDK JEP 491 proof that this module's docs cross-link to
instead of duplicating.

virtual-threads-benchmark/pom.xml

@@ -0,0 +1,48 @@
+<?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>
+
+  <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-boot4-demo</artifactId>
+  <version>1.0.0</version>
+  <name>virtual-threads-boot4-demo</name>
+  <description>Virtual threads on Spring Boot 4.1 / JDK 25, re-benchmarked, with a corrected pinning-diagnosis section for JEP 491</description>
+
+  <properties>
+    <java.version>25</java.version>
+  </properties>
+
+  <dependencies>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-web</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>
+  </dependencies>
+
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.springframework.boot</groupId>
+        <artifactId>spring-boot-maven-plugin</artifactId>
+      </plugin>
+    </plugins>
+  </build>
+</project>