Add storage-backed story history

src/engine/ink-engine.ts

@@ -109,12 +109,27 @@ export class InkEngine {
     if (!this.story) {
       throw new Error('No active Ink story to save');
     }
-    return this.story.state.toJson();
+    return JSON.stringify({
+      inkState: this.story.state.toJson(),
+      nextTurnId: this.nextTurnId,
+    });
   }
 
   loadGame(savedState: string): TurnResult {
     this.story = this.loadStory();
-    this.story.state.LoadJson(savedState);
+    let inkState = savedState;
+    try {
+      const parsed = JSON.parse(savedState);
+      if (parsed && typeof parsed.inkState === 'string') {
+        inkState = parsed.inkState;
+        if (Number.isInteger(parsed.nextTurnId)) {
+          this.nextTurnId = Math.max(1, parsed.nextTurnId);
+        }
+      }
+    } catch {
+      // Backward compatibility with raw Ink state JSON.
+    }
+    this.story.state.LoadJson(inkState);
     return this.continueStory();
   }