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/scripts/run.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+# Starts the app on :8080 to poke at by hand. Pass "vt" to enable virtual threads.
+#   ./scripts/run.sh         -> platform threads (Boot default)
+#   ./scripts/run.sh vt      -> spring.threads.virtual.enabled=true
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+VT_FLAG="false"
+if [ "${1:-}" = "vt" ]; then
+  VT_FLAG="true"
+fi
+
+mvn -q -B spring-boot:run -Dspring-boot.run.arguments="--spring.threads.virtual.enabled=${VT_FLAG}"