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/src/test/java/com/ankurm/brokers/Capture.java

@@ -0,0 +1,23 @@
+package com.ankurm.brokers;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+/** Writes a transcript under docs/output/. Nothing in the article is typed by hand. */
+public final class Capture {
+
+    private Capture() {
+    }
+
+    public static void write(String fileName, String heading, String body) {
+        Path dir = Path.of(System.getProperty("user.dir"), "docs", "output");
+        try {
+            Files.createDirectories(dir);
+            Files.writeString(dir.resolve(fileName), "== " + heading + " ==\n\n" + body + "\n");
+        }
+        catch (IOException ex) {
+            throw new IllegalStateException("could not write " + fileName, ex);
+        }
+    }
+}