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/05-what-helps.txt

@@ -0,0 +1,32 @@
+variant                            | Started in (s), each run   |   median
+-----------------------------------+----------------------------+---------
+no tracking                        | 6.871,6.456,7.473,6.992    |   6.9315
+BufferingApplicationStartup        | 6.448,6.856,6.416,6.528    |    6.488
+FlightRecorder + recording         | 6.496,6.739,6.343,6.475    |   6.4855
+lazy-initialization                | 5.075,5.331,5.217,5.266    |   5.2415
+
+=== JDK 25 AOT cache (Project Leyden) ===
+openjdk version "25.0.4.1" 2026-08-18 LTS
+-- training run (-XX:AOTMode=record) --
+-rw-r--r-- 1 eloquent-blissful-maxwell eloquent-blissful-maxwell 119484416 Sep  4 23:44 /tmp/app.aotconf
+-- assembly run (-XX:AOTMode=create) --
+Reading AOTConfiguration /tmp/app.aotconf and writing AOTCache /tmp/app.aot
+AOTCache creation is complete: /tmp/app.aot 118001664 bytes
+-rw-r--r-- 1 eloquent-blissful-maxwell eloquent-blissful-maxwell 118001664 Sep  4 23:44 /tmp/app.aot
+
+AOT cache (-XX:AOTCache)           | 5.028,4.729,4.708,4.905    |    4.817
+AOT cache + lazy                   | 3.649,3.488,3.354,3.384    |    3.436
+
+=== where the AOT cache takes the time from (same tracker, same jar) ===
+run                    |  parse ms |   inst ms | webserver
+-----------------------+-----------+-----------+----------
+plain                  |    986.66 |   4344.21 |    150.69
+aotcache               |    729.16 |   3155.96 |    100.96
+
+Notes:
+ * Timings are from one machine (Temurin 25.0.4.1+1, Linux x86_64, container) and are
+   indicative. The ratios between rows are the point, not the absolute numbers.
+ * "no tracking" and the two instrumented rows overlap. Recording startup steps is not
+   measurably slower than not recording them on this application.
+ * The AOT cache was trained on this exact jar with -XX:AOTMode=record and assembled with
+   -XX:AOTMode=create. It is 118 MB and is invalidated by a classpath change.