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.

src/main/java/com/ankurm/vt/Demo1PlainThreadLocal.java

@@ -0,0 +1,44 @@
+package com.ankurm.vt;
+
+// Explained in docs/01-inheritable-threadlocal.md -- run via scripts/run-all.sh, output captured in docs/output/
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.TimeUnit;
+
+/** Confirms, with no Spring involved, how InheritableThreadLocal behaves for
+ *  (a) a fresh platform Thread, (b) a reused thread from a fixed pool, and
+ *  (c) a fresh virtual thread. */
+public class Demo1PlainThreadLocal {
+
+    static final InheritableThreadLocal<String> CTX = new InheritableThreadLocal<>();
+
+    public static void main(String[] args) throws Exception {
+        System.out.println("=== Demo 1: InheritableThreadLocal across thread models ===");
+
+        // (a) fresh platform Thread inherits at construction time
+        CTX.set("request-A");
+        Thread t = new Thread(() -> System.out.println("fresh platform thread sees: " + CTX.get()));
+        t.start();
+        t.join();
+
+        // (b) reused thread from a fixed pool: the SECOND task on the same worker
+        // still carries whatever was set when the pool thread was originally created
+        ExecutorService pool = Executors.newFixedThreadPool(1);
+        CTX.set("request-B");
+        pool.submit(() -> System.out.println("pool thread, task 1, sees: " + CTX.get())).get();
+        CTX.set("request-C"); // caller's context changed
+        pool.submit(() -> System.out.println("pool thread, task 2 (reused), sees: " + CTX.get()
+                + "  <-- stale, not request-C")).get();
+        pool.shutdown();
+
+        // (c) fresh virtual thread, never reused
+        CTX.set("request-D");
+        Thread vt = Thread.ofVirtual().start(() ->
+                System.out.println("fresh virtual thread sees: " + CTX.get()));
+        vt.join();
+
+        CTX.remove();
+    }
+}