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/demo-versions.sh

@@ -0,0 +1,16 @@
+#!/usr/bin/env bash
+# Resolved versions, from the build rather than from the documentation.
+set -uo pipefail
+source "$(dirname "$0")/env.sh"
+cd "$ROOT"
+echo "=== java -version ==="
+"$JAVA_HOME/bin/java" -version 2>&1 | grep -v 'JAVA_TOOL_OPTIONS\|Picked up'
+echo
+echo "=== mvn -version ==="
+mvn -version 2>&1 | head -1
+echo
+echo "=== mvn dependency:list (selected) ==="
+mvn -B dependency:list -DoutputFile=/dev/stdout -DincludeScope=runtime 2>/dev/null \
+  | grep -E 'spring-boot:|spring-core|spring-context|spring-beans|spring-data-jpa|hibernate-core|micrometer-core|tomcat-embed-core|h2:' \
+  | sed 's/^\[INFO\] *//' | sort -u
+true