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/08-groups-and-probes.txt

@@ -0,0 +1,65 @@
+### profile: groups
+
+--- baseline: upstream UP ---
+  GET /actuator/health                     HTTP 503
+  GET /actuator/health/liveness            HTTP 200
+  GET /actuator/health/readiness           HTTP 200
+  GET /actuator/health/startup             HTTP 200
+
+--- upstream DOWN (a third party is having an outage) ---
+  GET /actuator/health                     HTTP 503
+  GET /actuator/health/liveness            HTTP 200
+  GET /actuator/health/readiness           HTTP 503
+  GET /actuator/health/startup             HTTP 200
+
+  liveness stayed 200. readiness went 503.
+  Kubernetes takes this instance out of the Service and leaves the process alone.
+  Wire readiness to /actuator/health and you get a restart loop instead.
+
+--- what each group contains ---
+$ curl -s http://localhost:8080/actuator/health/liveness
+{
+    "components": {
+        "diskSpace": {
+            "details": {
+                "total": 10213466112,
+                "free": 3877605376,
+                "threshold": 10485760,
+                "path": "/tmp/work/spring-boot-demo/.",
+                "exists": true
+            },
+            "status": "UP"
+        },
+        "livenessState": {
+            "status": "UP"
+        }
+    },
+    "status": "UP"
+}
+$ curl -s http://localhost:8080/actuator/health/readiness
+{
+    "components": {
+        "externalApi": {
+            "details": {
+                "error": "org.springframework.web.client.HttpServerErrorException$ServiceUnavailable: 503 : \"upstream unavailable\"",
+                "url": "http://localhost:8080/stub/upstream/ping",
+                "latencyMs": 66,
+                "timeoutMs": 750
+            },
+            "status": "DOWN"
+        },
+        "ordersDatabase": {
+            "details": {
+                "orders": 3,
+                "queryMs": 0,
+                "slowThresholdMs": 250
+            },
+            "status": "UP"
+        },
+        "readinessState": {
+            "status": "UP"
+        }
+    },
+    "status": "DOWN"
+}
+