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.

async/docs/output/pool-unbounded-queue.txt

@@ -0,0 +1,6 @@
+== core-size=4, max-size=12, queue-capacity=<unbounded default>, 16 blocking tasks ==
+
+distinct threads that ran a task : 4
+thread names                     : [task-1, task-2, task-3, task-4]
+max-size had no effect: the queue never refused a task.
+