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/04-scan-tax.txt

@@ -0,0 +1,10 @@
+runs per variant: 3   (first run of each also queried for phase self times)
+
+variant                    | Started in (s)     |  parse ms |    instMs |  steps
+---------------------------+--------------------+-----------+-----------+-------
+baseline                   | 7.293,6.685,6.86   |   1055.69 |   4854.73 |    400
++5000 plain classes        | 7.626,7.143,7.04   |   1617.38 |   4468.31 |    400
++5000 @Component           | 11.02,11.647,11.481 |   3221.82 |   6097.98 |   5400
++5000 @Component, lazy     | 9.688,9.374,9.609  |   3434.76 |   4982.61 |    324
+
+bulk package removed; jar rebuilt at baseline.