Add a ghost state tracking playground example

Co-authored-by: Codex <codex@openai.com>

Author: rcosta358

playground/index.md

@@ -21,6 +21,7 @@ Run the LiquidJava verification directly in your browser. Edit an example and se
           <option value="bounds">Parameter bounds</option>
           <option value="alias">Refinement aliases</option>
           <option value="state">Object states</option>
+          <option value="ghost">Ghost state tracking</option>
         </select>
       </div>
       <button id="lj-reset" type="button">Reset</button>

scripts/playground/examples.mjs

@@ -47,5 +47,26 @@ class Example {
         resource.read();
     }
 }
+`,
+  ghost: `import liquidjava.specification.*;
+
+@Ghost("int count")
+class Example {
+    @StateRefinement(to="count(this) == 0")
+    Example() {}
+
+    @StateRefinement(to="count(this) == count(old(this)) + 1")
+    void increment() {}
+
+    @StateRefinement(from="count(this) > 0", to="count(this) == count(old(this)) - 1")
+    void decrement() {}
+
+    static void demo() {
+        Example counter = new Example();
+        counter.increment();
+        counter.decrement();
+        counter.decrement();
+    }
+}
 `
 };