feat: isNewerMajor + store write-refusal backstop (F5 SLICE-2, INV-16, #14)

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

test/model.test.ts

@@ -2,6 +2,7 @@ import { describe, it, expect } from "vitest";
 import {
   SCHEMA_VERSION,
   emptyArtifact,
+  isNewerMajor,
   serializeArtifact,
   type Artifact,
 } from "../src/model";
@@ -103,6 +104,13 @@ describe("cross-rung identity (F5 SLICE-2)", () => {
   });
 });
 
+describe("isNewerMajor (F5 SLICE-2, INV-16)", () => {
+  it("is false for v1 and true for a newer major", () => {
+    expect(isNewerMajor(emptyArtifact("d.md"))).toBe(false);
+    expect(isNewerMajor({ schemaVersion: 2 })).toBe(true);
+  });
+});
+
 describe("unknown-field preservation (F5 SLICE-2, INV-15)", () => {
   it("a rewrite preserves unknown fields at every level, after known keys, sorted", () => {
     const foreign = {