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/scripts/gen-bulk.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+# gen-bulk.sh <count> <plain|component>
+# Writes <count> classes into the package that @SpringBootApplication already scans.
+#
+# 'plain' classes carry no stereotype annotation at all. They are still opened, read and
+# have their annotation metadata parsed by the scanner -- which is the point: it separates
+# the cost of *scanning* from the cost of *creating beans*.
+set -euo pipefail
+COUNT="${1:-5000}"
+KIND="${2:-plain}"
+ROOT="$(cd "$(dirname "$0")/.." && pwd)"
+DIR="$ROOT/src/main/java/com/ankurm/startup/bulk"
+
+rm -rf "$DIR"; mkdir -p "$DIR"
+for i in $(seq 1 "$COUNT"); do
+  if [ "$KIND" = "component" ]; then
+    printf 'package com.ankurm.startup.bulk;\nimport org.springframework.stereotype.Component;\n@Component\npublic class Bulk%d { public int id() { return %d; } }\n' "$i" "$i" > "$DIR/Bulk$i.java"
+  else
+    printf 'package com.ankurm.startup.bulk;\npublic class Bulk%d { public int id() { return %d; } }\n' "$i" "$i" > "$DIR/Bulk$i.java"
+  fi
+done
+echo "generated $COUNT $KIND classes in src/main/java/com/ankurm/startup/bulk"