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.

spring-boot-startup-time/scripts/stop.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+# Stop the demo application.
+#
+# Do NOT match on the main class name with `pkill -f`. Any shell whose command line
+# happens to contain that string -- including the one that wrote this script -- matches
+# too, and gets killed. Match on the executable being java AND the jar name instead.
+for p in $(ps -eo pid=,comm=,args= | awk '$2 == "java" && /startup-diagnosis-1\.0\.0\.jar/ { print $1 }'); do
+  kill -9 "$p" 2>/dev/null || true
+done
+sleep 1