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/docs/output/09-kafka-timeout.txt.tuned

@@ -0,0 +1,15 @@
+### Kafka health check against an unreachable broker -- tuned
+
+$ time curl -s http://localhost:8080/actuator/health/kafka
+{
+    "details": {
+        "bootstrap": "localhost:9092",
+        "tuned": true,
+        "error": "TimeoutException: null",
+        "probeMs": 1500,
+        "budgetMs": 1500
+    },
+    "status": "DOWN"
+}
+
+wall clock: 1.6 s