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/docs/output/06-buffer-overflow.txt

@@ -0,0 +1,28 @@
+--- capacity 2048 ---
+  started, recorded steps: 2048
+--- capacity 16384 ---
+  started, recorded steps: 5400
+
+=== which steps survive truncation (capacity 2048, 5400 available) ===
+
+  $ curl -s localhost:8080/actuator/startup | python3 -c '...'
+  events: 2048
+  first 3: ['spring.boot.application.starting',
+            'spring.boot.application.environment-prepared',
+            'spring.boot.application.context-prepared']
+  last 3 : ['spring.beans.instantiate', 'spring.beans.instantiate', 'spring.beans.instantiate']
+  last tags: [{'key': 'beanName', 'value': 'bulk2718'}]
+  names:   [('spring.beans.instantiate', 2029),
+            ('spring.context.bean-factory.post-process', 9),
+            ('spring.boot.application.starting', 1),
+            ('spring.boot.application.environment-prepared', 1)]
+
+The buffer keeps the FIRST 2048 steps and drops everything after. The timeline therefore
+ends mid-instantiation, and 'spring.context.refresh', 'spring.boot.application.started'
+and 'spring.boot.application.ready' -- the steps that bracket everything else -- are
+absent, because a step is only recorded when it ENDS.
+
+  $ grep -ic buffer app.log
+  0
+
+No warning. No exception. The endpoint returns 200 with a well-formed, truncated answer.