Add lazy-constants module: LazyConstant (JEP 531) against the holder idiom and double-checked locking

Basics, failure semantics on 26 vs 27, lazy collections, API across 25/26/27, preview-flag traps and a JMH read-path benchmark (jars fetched and sha1-checked, not committed).

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01TF9JWFvJSNm6HVzswzZU5a

lazy-constants/src/OldWays.java

@@ -0,0 +1,35 @@
+// The three classic ways to hold a value that is expensive to build. Plain Java, no preview features.
+public class OldWays {
+    static class Expensive {
+        final String how;
+        Expensive(String how) { this.how = how; System.out.println("  [init] building Expensive via " + how); }
+    }
+
+    // 1. Eager: built when OldWays is initialised, which is before main runs, whether or not anyone uses it.
+    static final Expensive EAGER = new Expensive("eager static final");
+
+    // 2. Holder idiom: the JVM initialises Holder the first time it is touched, and does so exactly once.
+    static class Holder { static final Expensive VALUE = new Expensive("holder idiom"); }
+    static Expensive holder() { return Holder.VALUE; }
+
+    // 3. Double-checked locking: correct only because the field is volatile.
+    private static volatile Expensive dcl;
+    static Expensive doubleChecked() {
+        Expensive local = dcl;
+        if (local == null) {
+            synchronized (OldWays.class) {
+                local = dcl;
+                if (local == null) dcl = local = new Expensive("double-checked locking");
+            }
+        }
+        return local;
+    }
+
+    public static void main(String[] args) {
+        System.out.println("main starts (the eager value already exists)");
+        System.out.println("first holder() call:");   holder();
+        System.out.println("second holder() call:");  holder();
+        System.out.println("first doubleChecked() call:");  doubleChecked();
+        System.out.println("second doubleChecked() call:"); doubleChecked();
+    }
+}