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/visibility.txt

@@ -0,0 +1,9 @@
+== @Async against final, protected and private methods ==
+
+proxy class                        : com.ankurm.async.VisibilityService$$SpringCGLIB$$0
+isCglibProxy                       : true
+publicMethod()                     : task-4 (virtual=false)
+finalMethod()                      : main (virtual=false)
+protectedMethod() via the proxy    : task-5 (virtual=false)
+callProtectedInternally()          : main (virtual=false)
+