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/docs/output/footprint.txt

@@ -0,0 +1,40 @@
+== Operational footprint, measured on one 2-core / 3.8 GB Linux box ==
+
+Each broker started from its shipped distribution with default configuration and a
+700 MB heap cap, on Temurin JDK 21. Times are one sample on a small box: treat them as
+orders of magnitude, not as a benchmark.
+
+broker                      : kafka
+time from launch to first
+  accepted connection       : 10035 ms
+resident memory at idle     : 333 MB
+server processes            : 1
+listening ports             : 9092 (broker), 9093 (controller)
+unpacked distribution size  : 135 MB
+settings in shipped default
+  configuration file        : 24   (server.properties)
+
+broker                      : rabbit
+time from launch to first
+  accepted connection       : 15053 ms
+resident memory at idle     : 115 MB
+server processes            : 1
+listening ports             : 5672 (AMQP), 4369 (epmd), 25672 (inter-node)
+unpacked distribution size  : 26 MB
+settings in shipped default
+  configuration file        : 0   (rabbitmq.conf.example)
+epmd processes            : 1
+NOTE: RabbitMQ ships no configuration file at all. rabbitmq.conf.example is
+      entirely commented out and the boot log records "Config file(s): (none)".
+      Everything in that file is documentation, not a default.
+
+broker                      : pulsar
+time from launch to first
+  accepted connection       : 20801 ms
+resident memory at idle     : 610 MB
+server processes            : 1
+listening ports             : 6650 (binary), 8080 (admin/REST), 2181 (ZooKeeper)
+unpacked distribution size  : 344 MB
+settings in shipped default
+  configuration file        : 357   (standalone.conf)
+