feat: crossrung-reply stand-in writer — validate, append, stable-serialize (F5 SLICE-4, #14)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/crossrungReply.test.ts

@@ -0,0 +1,70 @@
+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 { execFileSync } from "node:child_process";
+import { emptyArtifact, serializeArtifact, type Artifact } from "../src/model";
+import { validateSidecar } from "./helpers/validateSidecar";
+
+const SCRIPT = path.resolve(__dirname, "../scripts/crossrung-reply.mjs");
+let dir: string;
+let sidecar: string;
+
+beforeEach(() => {
+  dir = fs.mkdtempSync(path.join(os.tmpdir(), "cowriting-crossrung-"));
+  sidecar = path.join(dir, "sample.md.json");
+  const a = emptyArtifact("docs/sample.md");
+  a.anchors["a_1"] = { fingerprint: { text: "alpha", before: "", after: "", lineHint: 0 } };
+  a.threads.push({
+    id: "t_1",
+    anchorId: "a_1",
+    status: "open",
+    messages: [{ id: "m_1", author: { kind: "human", id: "ben" }, body: "hi", createdAt: "2026-06-10T00:00:00.000Z" }],
+  });
+  // INV-15 must hold through a FOREIGN writer too
+  (a as unknown as Record<string, unknown>).futureSection = [{ id: "f_1" }];
+  fs.writeFileSync(sidecar, serializeArtifact(a), "utf8");
+});
+afterEach(() => fs.rmSync(dir, { recursive: true, force: true }));
+
+function run(...args: string[]): string {
+  return execFileSync(process.execPath, [SCRIPT, ...args], { encoding: "utf8" });
+}
+
+describe("crossrung-reply stand-in (F5 SLICE-4, PUC-2)", () => {
+  it("appends a conforming reply, preserves unknowns, byte-identical to the reference serializer", () => {
+    run(sidecar, "t_1", "Reply from the forge", "--author-id", "gitea", "--author-email", "forge@example.org");
+    const raw = fs.readFileSync(sidecar, "utf8");
+    const parsed = JSON.parse(raw) as Artifact;
+    expect(parsed.threads[0].messages).toHaveLength(2);
+    const reply = parsed.threads[0].messages[1];
+    expect(reply.body).toBe("Reply from the forge");
+    expect(reply.author).toEqual({ kind: "human", id: "gitea", email: "forge@example.org" });
+    expect(reply.id).toMatch(/^m_/);
+    expect((parsed as unknown as Record<string, unknown>).futureSection).toEqual([{ id: "f_1" }]);
+    expect(validateSidecar(parsed).valid).toBe(true);
+    // the stand-in's independent serialization is byte-identical to the
+    // editor's (INV-2 across writers) — the contract-drift tripwire.
+    expect(serializeArtifact(parsed)).toBe(raw);
+  });
+
+  it("refuses an unknown thread id and leaves the file untouched", () => {
+    const before = fs.readFileSync(sidecar, "utf8");
+    expect(() => run(sidecar, "t_nope", "x")).toThrow();
+    expect(fs.readFileSync(sidecar, "utf8")).toBe(before);
+  });
+
+  it("refuses an invalid sidecar (validates BEFORE writing)", () => {
+    fs.writeFileSync(sidecar, '{"schemaVersion":1}\n', "utf8");
+    expect(() => run(sidecar, "t_1", "x")).toThrow();
+  });
+
+  it("refuses a newer-major sidecar (INV-16 holds for foreign writers too)", () => {
+    const v2 = JSON.parse(fs.readFileSync(sidecar, "utf8"));
+    v2.schemaVersion = 2;
+    fs.writeFileSync(sidecar, JSON.stringify(v2, null, 2) + "\n", "utf8");
+    const before = fs.readFileSync(sidecar, "utf8");
+    expect(() => run(sidecar, "t_1", "x")).toThrow();
+    expect(fs.readFileSync(sidecar, "utf8")).toBe(before);
+  });
+});