Add the async module

Every @Async behaviour that surprises people, asserted by a test and captured
to docs/output/: the self-invocation trap, what CGLIB cannot override, the
IllegalArgumentException a plain return type throws, the unbounded queue that
makes max-size decoration, spring.task.execution.propagate-context (new in
Boot 4.1.0), the two Executor beans that leave @Async on an unpooled
SimpleAsyncTaskExecutor, and JEP 491 measured on JDK 21 against JDK 25.

README.md

@@ -0,0 +1,25 @@
+# spring-async-demo
+
+Companion code for the asynchronous execution series on [ankurm.com](https://ankurm.com). Each
+directory is a self-contained Maven project for one article, with its own `pom.xml`, its own
+numbered documentation chapters, and its own captured output under `docs/output/` — regenerated
+by that module's `scripts/run-all.sh`, never typed by hand.
+
+| Module | Article | What it demonstrates |
+|---|---|---|
+| [`async/`](async/README.md) | [@Async in Spring Boot 4: Executors, Virtual Threads and the Self-Invocation Trap](https://ankurm.com/spring-boot-4-async-executors-virtual-threads/) | Which thread a method actually ran on, in every case where the answer is not the one you expect |
+
+## Common ground
+
+All modules target the same verified stack: **JDK 25** (Temurin 25.0.4.1+1), **Spring Boot
+4.1.1**, **Spring Framework 7.0.9**. Versions were read from `maven-metadata.xml` on Maven Central
+and from Boot's own `spring-boot-dependencies` POM, rather than from release announcements.
+
+Everything is asserted by a test and captured to a file. The measurement is nearly always the same
+one: the name of the thread that ran the work, returned by the code itself. Timing cannot tell a
+fast synchronous call from an asynchronous one, which is why `@Async` failures survive so long in
+production.
+
+## Licence
+
+MIT — see [LICENSE](LICENSE).