Add jdk28-zgc: ZGC startup and warmup baseline on JDK 25/27/28-ea, and a probe for JEP 545/546

Spring Boot 4.1.1 harness measuring first-response time versus initial heap size and a
20,000-request warmup. Shows -XX:+ZAdaptiveHeapSizing is not yet in any build.

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01WzVQ6zFh9cuLPj9RSbRH8U

jdk28-zgc/src/main/java/com/ankurm/zgc/DemoApplication.java

@@ -0,0 +1,43 @@
+package com.ankurm.zgc;
+
+import java.lang.management.ManagementFactory;
+import java.util.LinkedHashMap;
+import java.util.Map;
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+/** Smallest useful server: one cheap endpoint, one endpoint that reports heap state. */
+@SpringBootApplication
+public class DemoApplication {
+
+    public static void main(String[] args) {
+        SpringApplication.run(DemoApplication.class, args);
+    }
+
+    @RestController
+    static class Api {
+        private final byte[][] garbage = new byte[64][];
+        private int i;
+
+        /** Allocates a little so ZGC has something to do. */
+        @GetMapping("/hello")
+        String hello() {
+            garbage[i++ & 63] = new byte[16 * 1024];
+            return "ok";
+        }
+
+        @GetMapping("/heap")
+        Map<String, Object> heap() {
+            Runtime rt = Runtime.getRuntime();
+            Map<String, Object> m = new LinkedHashMap<>();
+            m.put("javaVersion", System.getProperty("java.version"));
+            m.put("committedMiB", rt.totalMemory() >> 20);
+            m.put("maxMiB", rt.maxMemory() >> 20);
+            m.put("gc", ManagementFactory.getGarbageCollectorMXBeans().stream().map(b -> b.getName()).toList());
+            m.put("jvmUptimeMs", ManagementFactory.getRuntimeMXBean().getUptime());
+            return m;
+        }
+    }
+}