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-versions.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+# Exactly what this repository was built and run against.
+set -uo pipefail
+cd "$(dirname "$0")/.."
+OUT=docs/output/00-versions.txt
+{
+  echo "\$ java -version"
+  java -version 2>&1 | grep -viE 'JAVA_TOOL_OPTIONS|Picked up'
+  echo
+  echo "\$ mvn -v | head -3"
+  mvn -v 2>&1 | grep -viE 'WARNING|sun\.misc|Picked up|Please consider' | head -3
+  echo
+  echo "\$ mvn dependency:list  -- the resolved versions behind spring-boot-starter-parent 4.1.1"
+  mvn -B dependency:list 2>/dev/null \
+    | grep -oE '(org\.springframework[a-z.]*|io\.micrometer|org\.apache\.kafka|com\.h2database):[A-Za-z0-9.-]+:jar:[0-9][A-Za-z0-9.-]*' \
+    | awk -F':' '{printf "  %-52s %s\n", $1":"$2, $4}' | sort -u \
+    | grep -E 'spring-boot-actuator |spring-boot-health |spring-boot |spring-core |spring-web |spring-security-core |spring-security-web |micrometer-core |micrometer-registry-prometheus |kafka-clients |h2 |spring-boot-restclient '
+} > "$OUT" 2>&1
+echo "wrote $OUT"