Add virtual-thread, structured-concurrency demos for the Spring Security context propagation guide

Three verified programs (JDK 25, Spring Security 7.1.1, Spring Boot 4.1.1 dependency
versions) backing the ankurm.com post: InheritableThreadLocal across thread models,
@Async on a virtual-thread SimpleAsyncTaskExecutor (DelegatingSecurityContextExecutor
vs ContextPropagatingTaskDecorator), and StructuredTaskScope.fork() propagation.
Captured console output and docs chapters included.

scripts/run-all.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+# Compiles and runs all three demos, regenerating docs/output/*.txt.
+# Requires JDK 25 (StructuredTaskScope is a preview API through JDK 25/JEP 505).
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+mvn -q dependency:build-classpath -Dmdep.outputFile=cp.txt
+CP=$(cat cp.txt)
+
+rm -rf target/classes
+mkdir -p target/classes docs/output
+javac --release 25 --enable-preview -cp "$CP" -d target/classes $(find src -name '*.java')
+
+for demo in Demo1PlainThreadLocal Demo2AsyncVirtualThreads Demo3StructuredConcurrency; do
+  num=$(echo "$demo" | grep -o '[0-9]')
+  out="docs/output/demo${num}.txt"
+  echo "Running $demo -> $out"
+  java --enable-preview -cp "target/classes:$CP" "com.ankurm.vt.$demo" | tee "$out"
+done