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.

actuator-in-production/scripts/demo-groups-probes.sh

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+# Health groups: keeping a dependency outage out of the liveness probe.
+set -uo pipefail
+cd "$(dirname "$0")/.."
+OUT=docs/output/08-groups-and-probes.txt
+B=http://localhost:8080/actuator/health
+{
+  echo "### profile: groups"
+  echo
+  echo "--- baseline: upstream UP ---"
+  for g in "" liveness readiness startup; do
+    u="$B${g:+/$g}"
+    printf '  %-40s HTTP %s\n' "GET ${u#http://localhost:8080}" "$(curl -s -o /dev/null -w '%{http_code}' -u ops:ops-password "$u")"
+  done
+  echo
+  echo "--- upstream DOWN (a third party is having an outage) ---"
+  curl -s -o /dev/null -u ops:ops-password -X POST "http://localhost:8080/stub/upstream/mode?value=down"
+  sleep 1
+  for g in "" liveness readiness startup; do
+    u="$B${g:+/$g}"
+    printf '  %-40s HTTP %s\n' "GET ${u#http://localhost:8080}" "$(curl -s -o /dev/null -w '%{http_code}' -u ops:ops-password "$u")"
+  done
+  echo
+  echo "  liveness stayed 200. readiness went 503."
+  echo "  Kubernetes takes this instance out of the Service and leaves the process alone."
+  echo "  Wire readiness to /actuator/health and you get a restart loop instead."
+  echo
+  echo "--- what each group contains ---"
+  echo "\$ curl -s $B/liveness"
+  curl -s -u ops:ops-password "$B/liveness" | python3 -m json.tool
+  echo "\$ curl -s $B/readiness"
+  curl -s -u ops:ops-password "$B/readiness" | python3 -m json.tool
+  echo
+  curl -s -o /dev/null -u ops:ops-password -X POST "http://localhost:8080/stub/upstream/mode?value=up"
+} > "$OUT" 2>&1
+echo "wrote $OUT"