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/01-default-exposure.txt

@@ -0,0 +1,35 @@
+### Spring Boot Actuator, starter added, ZERO management.* configuration
+
+$ curl -s -u ops:ops-password http://localhost:8080/actuator
+{
+    "_links": {
+        "self": {
+            "href": "http://localhost:8080/actuator",
+            "templated": false
+        },
+        "health": {
+            "href": "http://localhost:8080/actuator/health",
+            "templated": false
+        },
+        "health-path": {
+            "href": "http://localhost:8080/actuator/health/{*path}",
+            "templated": true
+        }
+    }
+}
+
+$ curl -s -o /dev/null -w '%{http_code}' -u ops:ops-password http://localhost:8080/actuator/env
+404
+  404 = discovered but NOT exposed over HTTP. Exposure and existence are different things.
+
+$ curl -s -u ops:ops-password http://localhost:8080/actuator/health
+{
+    "groups": [
+        "liveness",
+        "readiness"
+    ],
+    "status": "DOWN"
+}
+
+  Only 'health' is web-exposed by default. show-details defaults to 'never', so even an
+  authenticated caller sees a bare status until you say otherwise.