F2 SLICE-1: artifact schema + CoauthorStore with round-trip tests (#4)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/store.test.ts

@@ -0,0 +1,57 @@
+import { describe, it, expect, beforeEach, afterEach } from "vitest";
+import * as fs from "node:fs";
+import * as os from "node:os";
+import * as path from "node:path";
+import { CoauthorStore } from "../src/store";
+import { emptyArtifact, newId, serializeArtifact, type Artifact } from "../src/model";
+
+let root: string;
+
+beforeEach(() => {
+  root = fs.mkdtempSync(path.join(os.tmpdir(), "coauthor-store-"));
+});
+afterEach(() => {
+  fs.rmSync(root, { recursive: true, force: true });
+});
+
+function withThread(): Artifact {
+  const a = emptyArtifact("docs/spec.md");
+  a.anchors["a1"] = { fingerprint: { text: "hello", before: "", after: "", lineHint: 0 } };
+  a.threads.push({
+    id: "t1",
+    anchorId: "a1",
+    status: "open",
+    messages: [{ id: newId("m"), author: { kind: "human", id: "ben" }, body: "note", createdAt: "2026-06-10T00:00:00.000Z" }],
+  });
+  return a;
+}
+
+describe("CoauthorStore", () => {
+  it("returns null for a document with no sidecar", () => {
+    const store = new CoauthorStore(root);
+    expect(store.load("docs/spec.md")).toBeNull();
+  });
+
+  it("computes the sidecar path as .threads/<docPath>.json", () => {
+    const store = new CoauthorStore(root);
+    expect(store.sidecarPath("docs/spec.md")).toBe(path.join(root, ".threads", "docs", "spec.md.json"));
+  });
+
+  it("save then load round-trips the artifact", () => {
+    const store = new CoauthorStore(root);
+    const a = withThread();
+    store.save("docs/spec.md", a);
+    expect(store.load("docs/spec.md")).toEqual(a);
+  });
+
+  it("writes byte-stable, pretty JSON (re-save is identical)", () => {
+    const store = new CoauthorStore(root);
+    const a = withThread();
+    store.save("docs/spec.md", a);
+    const first = fs.readFileSync(store.sidecarPath("docs/spec.md"), "utf8");
+    store.save("docs/spec.md", store.load("docs/spec.md")!);
+    const second = fs.readFileSync(store.sidecarPath("docs/spec.md"), "utf8");
+    expect(second).toBe(first);
+    expect(first).toBe(serializeArtifact(a));
+  });
+});