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-what-helps.sh

@@ -0,0 +1,66 @@
+#!/usr/bin/env bash
+# The four things people reach for, measured on the same jar.
+#
+#   no tracking      : baseline, no ApplicationStartup installed
+#   buffering        : BufferingApplicationStartup(16384)   -- what it costs to measure
+#   jfr              : FlightRecorderApplicationStartup + an active recording
+#   lazy             : spring.main.lazy-initialization=true
+#   AOT cache        : JDK 25 ahead-of-time cache (JEP 483/515), trained on this app
+set -uo pipefail
+source "$(dirname "$0")/env.sh"
+RUNS="${RUNS:-4}"
+
+run_variant() {   # run_variant <label> <extra args...>
+  local label="$1"; shift
+  local out=()
+  for _ in $(seq 1 "$RUNS"); do
+    "$ROOT/scripts/stop.sh"
+    "$JAVA_HOME/bin/java" "$@" -jar "$JAR" > /tmp/helps.log 2>&1 &
+    local pid=$!
+    for _ in $(seq 1 240); do curl -fs -o /dev/null localhost:8080/actuator/health && break; sleep 0.5; done
+    out+=("$(grep -ho 'in [0-9.]* seconds' /tmp/helps.log | head -1 | awk '{print $2}')")
+    kill -9 $pid 2>/dev/null; wait $pid 2>/dev/null
+  done
+  # median of the runs
+  local med
+  med=$(printf '%s\n' "${out[@]}" | sort -n | awk '{a[NR]=$1} END {print (NR%2)? a[(NR+1)/2] : (a[NR/2]+a[NR/2+1])/2}')
+  printf '%-34s | %-26s | %8s\n' "$label" "$(IFS=,; echo "${out[*]}")" "$med"
+}
+
+printf '%-34s | %-26s | %8s\n' "variant" "Started in (s), each run" "median"
+printf -- '-----------------------------------+----------------------------+---------\n'
+
+run_variant "no tracking"        -Dstartup.tracking=none
+run_variant "BufferingApplicationStartup" -Dstartup.tracking=buffering
+run_variant "FlightRecorder + recording"   -Dstartup.tracking=jfr \
+            -XX:StartFlightRecording=filename=/tmp/helps.jfr,settings=profile,dumponexit=true
+run_variant "lazy-initialization"          -Dstartup.tracking=none -Dspring.profiles.active=lazy
+
+echo
+echo "=== JDK 25 AOT cache (Project Leyden) ==="
+"$JAVA_HOME/bin/java" -version 2>&1 | head -1
+rm -f /tmp/app.aotconf /tmp/app.aot
+
+echo "-- training run (-XX:AOTMode=record) --"
+"$JAVA_HOME/bin/java" -XX:AOTMode=record -XX:AOTConfiguration=/tmp/app.aotconf \
+  -Dstartup.tracking=none -jar "$JAR" > /tmp/aot-train.log 2>&1 &
+PID=$!
+for _ in $(seq 1 240); do curl -fs -o /dev/null localhost:8080/actuator/health && break; sleep 0.5; done
+curl -fs -o /dev/null localhost:8080/orders/summary || true
+kill -TERM $PID 2>/dev/null; wait $PID 2>/dev/null
+sleep 2
+ls -la /tmp/app.aotconf 2>/dev/null || { echo "no AOT configuration produced:"; tail -5 /tmp/aot-train.log; }
+
+echo "-- assembly run (-XX:AOTMode=create) --"
+"$JAVA_HOME/bin/java" -XX:AOTMode=create -XX:AOTConfiguration=/tmp/app.aotconf \
+  -XX:AOTCache=/tmp/app.aot -jar "$JAR" > /tmp/aot-create.log 2>&1
+tail -4 /tmp/aot-create.log
+ls -la /tmp/app.aot 2>/dev/null || echo "no cache produced"
+
+echo
+if [ -f /tmp/app.aot ]; then
+  run_variant "AOT cache (-XX:AOTCache)"  -XX:AOTCache=/tmp/app.aot -Dstartup.tracking=none
+  run_variant "AOT cache + lazy"          -XX:AOTCache=/tmp/app.aot -Dstartup.tracking=none -Dspring.profiles.active=lazy
+fi
+"$ROOT/scripts/stop.sh"
+true