feat(engine): coordinate notation and JSON save/load

src/engine/notation.test.ts

@@ -0,0 +1,23 @@
+import { describe, it, expect } from 'vitest';
+import { toCoordinate, serialize, deserialize } from './notation';
+import type { HistoryEntry } from './types';
+
+describe('notation', () => {
+  it('renders a move as a coordinate token', () => {
+    expect(toCoordinate({ from: 'e2', to: 'e4' })).toBe('e2e4');
+    expect(toCoordinate({ from: 'e7', to: 'e8', promotion: 'q' })).toBe('e7e8q');
+  });
+
+  it('round-trips a game through serialize/deserialize', () => {
+    const history: HistoryEntry[] = [
+      { player: 'N', from: 'e2', to: 'e4' },
+      { player: 'S', from: 'e2', to: 'e4' },
+    ];
+    const restored = deserialize(serialize(history));
+    expect(restored).toEqual(history);
+  });
+
+  it('rejects a file that is not a duplicate-chess save', () => {
+    expect(() => deserialize('{"variant":"chess","version":1,"moves":[]}')).toThrow();
+  });
+});