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

test/wallet.test.js

@@ -61,3 +61,27 @@ test("is idempotent by idempotency key", () => {
   assert.equal(first.id, second.id);
   assert.equal(wallet.getBalance("u1"), 4);
 });
+
+test("can restore state from previous snapshot", () => {
+  const original = new WalletStore();
+  original.applyTransaction({
+    userId: "u1",
+    type: "credit",
+    amount: 3,
+    reason: "topup",
+    idempotencyKey: "evt-r1",
+  });
+
+  const restored = new WalletStore(original.exportState());
+  assert.equal(restored.getBalance("u1"), 3);
+
+  const duplicate = restored.applyTransaction({
+    userId: "u1",
+    type: "credit",
+    amount: 3,
+    reason: "topup",
+    idempotencyKey: "evt-r1",
+  });
+  assert.equal(duplicate.amount, 3);
+  assert.equal(restored.getBalance("u1"), 3);
+});