Add kubernetes-deployment: probes, shutdown, JVM ergonomics, HPA

Companion code for "Deploying Spring Boot 4 on Kubernetes: Probes, Graceful
Shutdown, Limits and JVM Ergonomics". A dependency outage under three
probe-group setups, a rolling restart under load four ways (three runs
each), the JVM's ergonomic choices for nine pod shapes, one GC-heavy load
under five CPU limits with throttling counters, and an HPA driven by a
Micrometer gauge through prometheus-adapter. Measured on k3s v1.36.4.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01C3TETMrqVUWeFkNtz3Jbo3

kubernetes-deployment/k8s/client.yaml

@@ -0,0 +1,13 @@
+# A shell inside the cluster for the scripts to call Services from (the app image has none).
+apiVersion: v1
+kind: Pod
+metadata:
+  name: client
+  namespace: demo
+spec:
+  terminationGracePeriodSeconds: 1
+  containers:
+    - name: busybox
+      image: busybox:1.37
+      imagePullPolicy: Never
+      command: ["sh", "-c", "trap 'exit 0' TERM; while true; do sleep 1; done"]