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/env.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+# Shared environment. Point JAVA_HOME at a JDK 25 (or newer) installation.
+: "${JAVA_HOME:?set JAVA_HOME to a JDK 25+ installation}"
+export PATH="$JAVA_HOME/bin:$PATH"
+MVN="${MVN:-mvn}"
+APP_MAIN="com.ankurm.actuator.ActuatorProductionApplication"
+APP_PORT="${APP_PORT:-8080}"
+MGMT_PORT="${MGMT_PORT:-9001}"