feat: add engine and store snapshot/restore for restart resilience

test/engine.test.js

@@ -154,3 +154,29 @@ test("lists jobs for user newest first and provides summary", () => {
   assert.equal(summary.totalCreditsSpent, 2);
   assert.equal(summary.balance, 8);
 });
+
+test("round-trips state snapshot across engine restart", () => {
+  const engine1 = createEngine();
+  engine1.topUpCredits("u1", 5, "topup-snapshot");
+
+  const created = engine1.processMention({
+    mentionPostId: "m-snapshot",
+    callerUserId: "u1",
+    parentPost: {
+      id: "p-snapshot",
+      authorId: "author-snapshot",
+      article: { id: "a-snapshot", title: "Snap", body: "hello world" },
+    },
+  });
+
+  const snapshot = engine1.exportState();
+  const engine2 = new XArtAudioEngine({
+    creditConfig: engine1.creditConfig,
+    initialState: snapshot,
+  });
+
+  assert.equal(engine2.getWalletBalance("u1"), 4);
+  assert.equal(engine2.getJob(created.job.id).article.title, "Snap");
+  assert.equal(engine2.getAsset(created.job.assetId).articleTitle, "Snap");
+  assert.equal(engine2.checkAudioAccess(created.job.assetId, "u1").allowed, true);
+});