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-startup-tree.sh

@@ -0,0 +1,47 @@
+#!/usr/bin/env bash
+# Boots with BufferingApplicationStartup, then dumps the step tree three ways:
+# self time by phase, slowest steps by self time, and the same list by total time
+# (which is what /actuator/startup would have you sort by).
+set -uo pipefail
+source "$(dirname "$0")/env.sh"
+
+"$(dirname "$0")/stop.sh"
+"$JAVA_HOME/bin/java" -Dstartup.tracking=buffering -jar "$JAR" > /tmp/startup-app.log 2>&1 &
+PID=$!
+for _ in $(seq 1 180); do curl -fs -o /dev/null localhost:8080/actuator/health && break; sleep 0.5; done
+
+echo "=== Boot's own startup line ==="
+grep -h 'Started StartupDiagnosisApplication' /tmp/startup-app.log | sed 's/^.*: //'
+echo
+echo "=== /diag/startup ==="
+curl -s 'localhost:8080/diag/startup?top=12' > /tmp/diag.json
+python3 - <<'PY'
+import json
+d = json.load(open('/tmp/diag.json'))
+print(f"recorded steps: {d['recordedSteps']}\n")
+print("-- self time by step name (self = duration minus direct children) --")
+print(f"{'self ms':>9}  {'count':>5}  step")
+for r in d['phasesBySelfTime']:
+    print(f"{r['selfMs']:9.2f}  {r['count']:5d}  {r['step']}")
+def bean(t): return t.get('beanName') or t.get('classNames') or ''
+print("\n-- slowest individual steps by SELF time (the culprits) --")
+print(f"{'self ms':>9}  {'total ms':>9}  step / tags")
+for r in d['slowestBySelfTime']:
+    print(f"{r['selfMs']:9.2f}  {r['totalMs']:9.2f}  {r['name']} {bean(r['tags'])}")
+print("\n-- slowest individual steps by TOTAL time (the containers) --")
+print(f"{'total ms':>9}  {'self ms':>9}  step / tags")
+for r in d['slowestByTotalTime']:
+    print(f"{r['totalMs']:9.2f}  {r['selfMs']:9.2f}  {r['name']} {bean(r['tags'])}")
+PY
+
+echo
+echo "=== actuator's own endpoint: GET peeks, POST drains ==="
+for verb in GET POST POST GET; do
+  n=$(curl -s -X $verb localhost:8080/actuator/startup \
+      | python3 -c "import sys,json;print(len(json.load(sys.stdin).get('timeline',{}).get('events',[])))" 2>/dev/null || echo "-")
+  echo "  $verb  /actuator/startup -> events in response: $n"
+done
+
+kill -9 $PID 2>/dev/null
+wait $PID 2>/dev/null
+true