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/stop.sh

@@ -0,0 +1,14 @@
+#!/usr/bin/env bash
+# Kill by main class, never by a pattern that could match this script's own command line.
+# `pkill -f spring-boot` matches the shell running it and takes the terminal with it.
+set -u
+for p in $(ps -eo pid,args | grep '[A]ctuatorProductionApplication' | awk '{print $1}'); do
+  kill -9 "$p" 2>/dev/null || true
+done
+# Wait for the port to actually close. Killing the PID is not the same as the socket being
+# free, and a stale listener looks exactly like your configuration change having no effect.
+for _ in $(seq 1 40); do
+  if ! (exec 3<>/dev/tcp/127.0.0.1/"${APP_PORT:-8080}") 2>/dev/null; then break; fi
+  sleep 0.25
+done
+exec 3<&- 2>/dev/null || true