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/run.sh

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+# run.sh [profiles] [tracking]   e.g. ./scripts/run.sh lazy buffering
+set -euo pipefail
+source "$(dirname "$0")/env.sh"
+"$(dirname "$0")/stop.sh"
+
+PROFILES="${1:-}"
+TRACKING="${2:-buffering}"
+ARGS=(-Dstartup.tracking="$TRACKING")
+[ -n "$PROFILES" ] && ARGS+=(-Dspring.profiles.active="$PROFILES")
+
+setsid nohup "$JAVA_HOME/bin/java" "${ARGS[@]}" -jar "$JAR" > /tmp/startup-app.log 2>&1 < /dev/null &
+for _ in $(seq 1 120); do
+  curl -fs -o /dev/null http://localhost:8080/actuator/health && break
+  sleep 0.5
+done
+echo "up: profiles='${PROFILES}' tracking='${TRACKING}' (log: /tmp/startup-app.log)"