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-scan-tax.sh

@@ -0,0 +1,71 @@
+#!/usr/bin/env bash
+# Quantifies what component scanning costs, by adding classes to the scanned package
+# and changing nothing else.
+#
+#   baseline    : the application as committed
+#   +5000 plain : 5000 classes with no annotations -- pure scan cost
+#   +5000 @Component : the same 5000, now bean definitions -- scan + define + instantiate
+#   +5000 @Component, lazy : lazy init removes the instantiation, not the scan
+#
+# Each variant is a full rebuild. Budget about ten minutes.
+set -uo pipefail
+source "$(dirname "$0")/env.sh"
+cd "$ROOT"
+RUNS="${RUNS:-3}"
+
+measure() {   # measure <label> <extra java args...>
+  local label="$1"; shift
+  local started=() parse="" inst="" steps=""
+  for i in $(seq 1 "$RUNS"); do
+    "$ROOT/scripts/stop.sh"
+    "$JAVA_HOME/bin/java" -Dstartup.tracking=buffering "$@" -jar "$JAR" > /tmp/scan-app.log 2>&1 &
+    local pid=$!
+    for _ in $(seq 1 240); do curl -fs -o /dev/null localhost:8080/actuator/health && break; sleep 0.5; done
+    started+=("$(grep -ho 'in [0-9.]* seconds' /tmp/scan-app.log | head -1 | awk '{print $2}')")
+    if [ "$i" = "1" ]; then
+      curl -s 'localhost:8080/diag/startup?top=1' > /tmp/scan-diag.json
+      read -r parse inst steps <<<"$(python3 - <<'PY'
+import json
+d = json.load(open('/tmp/scan-diag.json'))
+p = {r['step']: r for r in d['phasesBySelfTime']}
+print(p.get('spring.context.config-classes.parse', {}).get('selfMs', 0),
+      p.get('spring.beans.instantiate', {}).get('selfMs', 0),
+      d['recordedSteps'])
+PY
+)"
+    fi
+    kill -9 $pid 2>/dev/null; wait $pid 2>/dev/null
+  done
+  printf '%-26s | %-18s | %9s | %9s | %6s\n' \
+    "$label" "$(IFS=,; echo "${started[*]}")" "$parse" "$inst" "$steps"
+}
+
+hdr() {
+  printf '%-26s | %-18s | %9s | %9s | %6s\n' \
+    "variant" "Started in (s)" "parse ms" "instMs" "steps"
+  printf -- '---------------------------+--------------------+-----------+-----------+-------\n'
+}
+
+echo "runs per variant: $RUNS   (first run of each also queried for phase self times)"
+echo
+hdr
+
+./scripts/gen-bulk.sh 0 plain >/dev/null 2>&1 || true
+rm -rf src/main/java/com/ankurm/startup/bulk
+mvn -B -q -DskipTests package > /tmp/scan-build.log 2>&1
+measure "baseline"
+
+./scripts/gen-bulk.sh 5000 plain > /dev/null
+mvn -B -q -DskipTests package > /tmp/scan-build.log 2>&1
+measure "+5000 plain classes"
+
+./scripts/gen-bulk.sh 5000 component > /dev/null
+mvn -B -q -DskipTests package > /tmp/scan-build.log 2>&1
+measure "+5000 @Component"
+measure "+5000 @Component, lazy" -Dspring.profiles.active=lazy
+
+rm -rf src/main/java/com/ankurm/startup/bulk
+mvn -B -q -DskipTests package > /tmp/scan-build.log 2>&1
+echo
+echo "bulk package removed; jar rebuilt at baseline."
+true