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/executor-report.txt

@@ -0,0 +1,8 @@
+== Stock Spring Boot 4.1.1 context, nothing configured ==
+
+Executor beans in this context: 1
+  applicationTaskExecutor  ->  org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor  [TaskExecutor]
+        corePoolSize=8 maxPoolSize=2147483647 queueCapacity=2147483647 threadNamePrefix=task-
+Bean named 'taskExecutor' present: false
+Bean named 'applicationTaskExecutor' present: true
+