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

src/engine/notation.ts

@@ -0,0 +1,25 @@
+import type { SyncMove, HistoryEntry } from './types';
+
+/** Coordinate notation, e.g. "e2e4" or "e7e8q". */
+export function toCoordinate(move: SyncMove): string {
+  return `${move.from}${move.to}${move.promotion ?? ''}`;
+}
+
+export interface SavedGame {
+  variant: 'duplicate-chess';
+  version: 1;
+  moves: HistoryEntry[];
+}
+
+export function serialize(history: HistoryEntry[]): string {
+  const data: SavedGame = { variant: 'duplicate-chess', version: 1, moves: history };
+  return JSON.stringify(data, null, 2);
+}
+
+export function deserialize(json: string): HistoryEntry[] {
+  const data = JSON.parse(json) as Partial<SavedGame>;
+  if (data.variant !== 'duplicate-chess' || !Array.isArray(data.moves)) {
+    throw new Error('Not a duplicate-chess save file');
+  }
+  return data.moves;
+}