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

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

src/store.ts

@@ -8,7 +8,7 @@
  */
 import * as fs from "node:fs";
 import * as path from "node:path";
-import { emptyArtifact, serializeArtifact, type Artifact } from "./model";
+import { SCHEMA_VERSION, emptyArtifact, isNewerMajor, serializeArtifact, type Artifact } from "./model";
 
 export class CoauthorStore {
   /**
@@ -67,6 +67,13 @@ export class CoauthorStore {
    */
   update(docPath: string, mutate: (artifact: Artifact) => void): Artifact {
     const artifact = this.load(docPath) ?? emptyArtifact(docPath);
+    if (isNewerMajor(artifact)) {
+      // INV-16 backstop: the controllers gate first (VersionGuard); this throw
+      // guarantees no missed write path can ever destroy a newer rung's data.
+      throw new Error(
+        `refusing to write ${docPath}: sidecar schemaVersion ${artifact.schemaVersion} > supported ${SCHEMA_VERSION} (INV-16)`,
+      );
+    }
     mutate(artifact);
     const referenced = new Set<string>([
       ...artifact.threads.map((t) => t.anchorId),