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/measure-startup.sh

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+# measure-startup.sh <runs> [-- extra java args...]
+# Starts the jar, waits for the "Started ... in Ns" line, records it, stops, repeats.
+set -uo pipefail
+source "$(dirname "$0")/env.sh"
+
+RUNS="${1:-5}"; shift || true
+[ "${1:-}" = "--" ] && shift
+
+for i in $(seq 1 "$RUNS"); do
+  LOG=$(mktemp)
+  "$JAVA_HOME/bin/java" "$@" -jar "$JAR" > "$LOG" 2>&1 &
+  PID=$!
+  for _ in $(seq 1 120); do
+    grep -q 'Started StartupDiagnosisApplication' "$LOG" && break
+    kill -0 $PID 2>/dev/null || break
+    sleep 0.25
+  done
+  grep -h 'Started StartupDiagnosisApplication' "$LOG" | sed 's/.*Started/Started/'
+  kill -9 $PID 2>/dev/null || true
+  wait $PID 2>/dev/null
+  rm -f "$LOG"
+  sleep 1
+done