Add the broker-comparison module

Kafka, RabbitMQ and Pulsar measured side by side on ordering, replay and
consumer scaling by driving the three client libraries directly, plus an
operational-footprint measurement of each broker's own distribution.

Nine tests, three brokers started without Docker, and every number in the
documentation regenerated by scripts/run-all.sh.

broker-comparison/scripts/run-all.sh

@@ -0,0 +1,37 @@
+#!/usr/bin/env bash
+# Regenerates every file under docs/output/. Each broker is started, measured and left running
+# only for its own group of tests, because three brokers do not fit comfortably in memory on a
+# small machine at the same time.
+#
+# Required environment:
+#   KAFKA_HOME     unpacked kafka_2.13-4.2.1
+#   ERL_ROOT       an Erlang 24 installation      (RabbitMQ 3.10.x)
+#   RABBITMQ_HOME  unpacked rabbitmq-server-generic-unix-3.10.25
+#   PULSAR_HOME    unpacked apache-pulsar-4.2.4-bin
+set -euo pipefail
+cd "$(dirname "$0")/.."
+mkdir -p docs/output
+
+# Kafka needs no external broker for the measurements: spring-kafka-test starts a real KRaft
+# broker in-process. KAFKA_HOME is only used by the footprint measurement.
+mvn -B -Dgroups=kafka test 2>&1 | grep -E 'Running |Tests run:|BUILD ' > docs/output/tests.txt
+
+scripts/rabbit-broker.sh
+mvn -B -Dgroups=rabbit test 2>&1 | grep -E 'Running |Tests run:|BUILD ' >> docs/output/tests.txt
+
+scripts/pulsar-broker.sh
+mvn -B -Dgroups=pulsar test 2>&1 | grep -E 'Running |Tests run:|BUILD ' >> docs/output/tests.txt
+
+{
+  echo "== Operational footprint, measured on one 2-core / 3.8 GB Linux box =="
+  echo
+  echo "Each broker started from its shipped distribution with default configuration and a"
+  echo "700 MB heap cap, on Temurin JDK 21. Times are one sample on a small box: treat them as"
+  echo "orders of magnitude, not as a benchmark."
+  echo
+  scripts/footprint.sh kafka;  echo
+  scripts/footprint.sh rabbit; echo
+  scripts/footprint.sh pulsar
+} > docs/output/footprint.txt
+
+echo "regenerated:"; ls -1 docs/output