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-buffer-overflow.sh

@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+# What a too-small buffer does. Every guide picks 2048; nobody says what happens when
+# an application produces more steps than that.
+set -uo pipefail
+source "$(dirname "$0")/env.sh"
+
+for cap in 2048 16384; do
+  "$(dirname "$0")/stop.sh"
+  "$JAVA_HOME/bin/java" -Dstartup.tracking=buffering -Dstartup.buffer=$cap \
+    -jar "$JAR" > /tmp/buf.log 2>&1 &
+  PID=$!
+  for _ in $(seq 1 240); do curl -fs -o /dev/null localhost:8080/actuator/health && break; sleep 0.5; done
+  ok=$?
+  echo "--- capacity $cap ---"
+  if grep -q 'Started StartupDiagnosisApplication' /tmp/buf.log; then
+    echo -n "  started, recorded steps: "
+    curl -s 'localhost:8080/diag/startup?top=1' \
+      | python3 -c "import sys,json;print(json.load(sys.stdin)['recordedSteps'])"
+  else
+    echo "  application did not start. Last lines of the log:"
+    grep -E 'ERROR|Exception|Caused by' /tmp/buf.log | head -5 | sed 's/^/    /'
+  fi
+  kill -9 $PID 2>/dev/null; wait $PID 2>/dev/null
+done
+true