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/self-invocation.txt

@@ -0,0 +1,7 @@
+== Which thread each call actually ran on (Boot 4.1.1, JDK 25) ==
+
+caller thread                : main (virtual=false)
+service.annotated()          : task-7 (virtual=false)
+service.viaSelfInvocation()  : main (virtual=false)
+service.viaSelfReference()   : task-8 (virtual=false)
+