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/kafka-ordering.txt

@@ -0,0 +1,30 @@
+== Kafka: ordering is per partition ==
+
+produced 12 records, keys D/A/F round-robin, values 1..12 in order
+(keys chosen so that murmur2 spreads them: D->0, A->1, F->2. A, B and C
+ all hash to partition 1 with three partitions, which is worth knowing
+ before you decide your keys are well distributed.)
+
+consumed in this order:
+  F=2@p2
+  F=5@p2
+  F=8@p2
+  F=11@p2
+  A=1@p1
+  A=4@p1
+  A=7@p1
+  A=10@p1
+  D=3@p0
+  D=6@p0
+  D=9@p0
+  D=12@p0
+
+per key:
+  F -> partition [2]  values [2, 5, 8, 11]
+  A -> partition [1]  values [1, 4, 7, 10]
+  D -> partition [0]  values [3, 6, 9, 12]
+
+Order is preserved within each key because a key hashes to one partition.
+Across keys it is not: the values above are not 1..12 in order, because a
+consumer drains one partition's buffer before the next.
+