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

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

src/model.ts

@@ -116,6 +116,14 @@ export function emptyArtifact(docPath: string): Artifact {
   };
 }
 
+/**
+ * INV-16 fail-safe: schemaVersion IS the major. A sidecar from a future major
+ * is render-only — a conforming writer never rewrites what it can't fully read.
+ */
+export function isNewerMajor(a: Pick<Artifact, "schemaVersion">): boolean {
+  return a.schemaVersion > SCHEMA_VERSION;
+}
+
 /** Stable, generated id (spec §6.3). */
 export function newId(prefix: string): string {
   return `${prefix}_${randomUUID()}`;