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-default-exposure.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+# What Actuator exposes when you add the starter and configure nothing.
+set -uo pipefail
+cd "$(dirname "$0")/.."
+OUT=docs/output/01-default-exposure.txt
+{
+  echo "### Spring Boot Actuator, starter added, ZERO management.* configuration"
+  echo
+  echo "\$ curl -s -u ops:ops-password http://localhost:8080/actuator"
+  curl -s -u ops:ops-password http://localhost:8080/actuator | python3 -m json.tool
+  echo
+  echo "\$ curl -s -o /dev/null -w '%{http_code}' -u ops:ops-password http://localhost:8080/actuator/env"
+  curl -s -o /dev/null -w '%{http_code}\n' -u ops:ops-password http://localhost:8080/actuator/env
+  echo "  404 = discovered but NOT exposed over HTTP. Exposure and existence are different things."
+  echo
+  echo "\$ curl -s -u ops:ops-password http://localhost:8080/actuator/health"
+  curl -s -u ops:ops-password http://localhost:8080/actuator/health | python3 -m json.tool
+  echo
+  echo "  Only 'health' is web-exposed by default. show-details defaults to 'never', so even an"
+  echo "  authenticated caller sees a bare status until you say otherwise."
+} > "$OUT" 2>&1
+echo "wrote $OUT"