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/docs/output/hpa-custom-metrics-api.txt

@@ -0,0 +1,27 @@
+# What Micrometer exports (one pod):
+$ curl <pod>:8080/actuator/prometheus | grep app_inflight
+# HELP app_inflight_requests Requests currently being processed by /work
+# TYPE app_inflight_requests gauge
+app_inflight_requests{application="orders"} 0.0
+
+# What the HPA controller sees through the custom metrics API:
+$ kubectl get --raw /apis/custom.metrics.k8s.io/v1beta1/namespaces/demo/pods/%2A/app_inflight_requests
+{
+    "kind": "MetricValueList",
+    "apiVersion": "custom.metrics.k8s.io/v1beta1",
+    "metadata": {},
+    "items": [
+        {
+            "describedObject": {
+                "kind": "Pod",
+                "namespace": "demo",
+                "name": "orders-7774f94779-gvc7k",
+                "apiVersion": "/v1"
+            },
+            "metricName": "app_inflight_requests",
+            "timestamp": "2026-09-11T16:33:28Z",
+            "value": "0",
+            "selector": null
+        }
+    ]
+}