fix: real project README and save-file version validation

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/engine/notation.test.ts

@@ -20,4 +20,8 @@ describe('notation', () => {
   it('rejects a file that is not a duplicate-chess save', () => {
     expect(() => deserialize('{"variant":"chess","version":1,"moves":[]}')).toThrow();
   });
+
+  it('rejects an unsupported save version', () => {
+    expect(() => deserialize('{"variant":"duplicate-chess","version":99,"moves":[]}')).toThrow();
+  });
 });

src/engine/notation.ts

@@ -21,5 +21,8 @@ export function deserialize(json: string): HistoryEntry[] {
   if (data.variant !== 'duplicate-chess' || !Array.isArray(data.moves)) {
     throw new Error('Not a duplicate-chess save file');
   }
+  if (data.version !== 1) {
+    throw new Error('Unsupported save version');
+  }
   return data.moves;
 }