feat: add atomic JSON file state persistence for production runtime

test/state-store.test.js

@@ -0,0 +1,35 @@
+"use strict";
+
+const test = require("node:test");
+const assert = require("node:assert/strict");
+const os = require("node:os");
+const path = require("node:path");
+const fs = require("node:fs/promises");
+const { JsonFileStateStore } = require("../src/lib/state-store");
+
+test("load returns null when state file does not exist", async () => {
+  const tempDir = await fs.mkdtemp(path.join(os.tmpdir(), "xartaudio-state-test-"));
+  const store = new JsonFileStateStore(path.join(tempDir, "missing.json"));
+
+  const data = await store.load();
+  assert.equal(data, null);
+});
+
+test("save and load roundtrip state", async () => {
+  const tempDir = await fs.mkdtemp(path.join(os.tmpdir(), "xartaudio-state-test-"));
+  const filePath = path.join(tempDir, "state.json");
+  const store = new JsonFileStateStore(filePath);
+
+  const expected = {
+    version: 1,
+    engine: {
+      jobs: { "1": { id: "1", status: "completed" } },
+      nextJobId: 2,
+    },
+  };
+
+  await store.save(expected);
+  const actual = await store.load();
+
+  assert.deepEqual(actual, expected);
+});