Spring Boot startup time: bean-by-bean diagnosis, and one directory per post

Adds spring-boot-startup-time/, the companion project for BLOG-618: a runnable
Spring Boot 4.1.1 application on JDK 25 that installs BufferingApplicationStartup
and FlightRecorderApplicationStartup behind a system property, and a /diag/startup
endpoint that computes step self time -- the number /actuator/startup does not give
you and the one that names the actual culprits.

Captured under docs/output/: the step tree sorted both ways, the same startup as JFR
events, a +5000-class experiment putting 0.11 ms per scanned class on the classpath
scan tax, the silent truncation a 2048-step buffer performs, and JDK 25 AOT cache
timings (6.93 s to 4.82 s). Post body and metadata live in post/.

Moves the existing Actuator project into actuator-in-production/ so the repository
holds one directory per article; the root README is now an index.

actuator-in-production/scripts/demo-kafka-timeout.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+# The single most useful number in this repository: how long a naive Kafka health check blocks.
+set -uo pipefail
+cd "$(dirname "$0")/.."
+OUT=docs/output/09-kafka-timeout.txt
+MODE="${1:-tuned}"
+B=http://localhost:8080/actuator/health/kafka
+{
+  echo "### Kafka health check against an unreachable broker -- ${MODE}"
+  echo
+  echo "\$ time curl -s $B"
+  s=$(date +%s.%N)
+  body=$(curl -s -u ops:ops-password --max-time 180 "$B")
+  e=$(date +%s.%N)
+  echo "$body" | python3 -m json.tool 2>/dev/null || echo "$body"
+  echo
+  printf 'wall clock: %.1f s\n' "$(echo "$e - $s" | bc)"
+} > "$OUT.$MODE" 2>&1
+echo "wrote $OUT.$MODE"