Java 27 and 26: runnable demos and captured output for every JEP, plus version lanes

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01B38FGKKam5SCGgwgduVAh3

lanes/upgrade/src/main/java/demo/App.java

@@ -0,0 +1,12 @@
+package demo;
+
+/**
+ * The smallest class that proves a build targets Java 25: it prints the Java version of the JVM it runs on.
+ * Explained in docs/15-build-files.md. The build files next to it (pom.xml, build.gradle.kts) are the ones
+ * quoted in the "Step 1" section of the upgrade guide.
+ */
+public class App {
+    public static void main(String[] args) {
+        System.out.println("running on Java " + Runtime.version().feature());
+    }
+}