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/scripts/pinning-probe.sh

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+# Runs PinningProbe with the virtual-thread scheduler limited to two carrier threads, so that
+# pinning (if it happened) would serialise the work into visible seconds.
+#
+# The probe is compiled on its own with --release 21 rather than as part of the module, so the
+# same class file can be run on a JDK 21 and a JDK 25 JVM. That comparison is the whole point:
+# JEP 491 landed in JDK 24, and the difference between the two runs is the feature.
+#
+# Usage: scripts/pinning-probe.sh <path-to-javac> <path-to-java> [more java binaries...]
+set -euo pipefail
+cd "$(dirname "$0")/.."
+JAVAC_BIN="${1:-javac}"
+shift || true
+OUT=target/probe-classes
+mkdir -p "$OUT"
+"$JAVAC_BIN" --release 21 -d "$OUT" src/main/java/com/ankurm/async/PinningProbe.java
+
+for JAVA_BIN in "${@:-java}"; do
+  echo "---------------------------------------------------------------"
+  "$JAVA_BIN" \
+    -Djdk.virtualThreadScheduler.parallelism=2 \
+    -Djdk.virtualThreadScheduler.maxPoolSize=2 \
+    -cp "$OUT" com.ankurm.async.PinningProbe
+done