feat(engine): ghost derivation from cross-board piece comparison

src/engine/ghosts.ts

@@ -0,0 +1,32 @@
+import type { DuplicateGame } from './game';
+import type { GhostMarker, BoardId, Color, Square } from './types';
+import { PLAYERS, PLAYER_BOARDS, PLAYER_COLOR } from './boards';
+
+/** Squares occupied by a piece of `color` on `board`. */
+function colorSquares(game: DuplicateGame, board: BoardId, color: Color): Set<Square> {
+  const set = new Set<Square>();
+  for (const row of game.boards[board].board()) {
+    for (const cell of row) {
+      if (cell && cell.color === color) set.add(cell.square);
+    }
+  }
+  return set;
+}
+
+/**
+ * Ghosts across all four players. A player's non-ghost pieces always occupy
+ * identical squares on both their boards (they move in lockstep), so a piece is
+ * a ghost iff the player's other board has no same-colour piece on that square.
+ */
+export function ghosts(game: DuplicateGame): GhostMarker[] {
+  const markers: GhostMarker[] = [];
+  for (const player of PLAYERS) {
+    const [a, b] = PLAYER_BOARDS[player];
+    const color = PLAYER_COLOR[player];
+    const sqA = colorSquares(game, a, color);
+    const sqB = colorSquares(game, b, color);
+    for (const sq of sqA) if (!sqB.has(sq)) markers.push({ board: a, square: sq });
+    for (const sq of sqB) if (!sqA.has(sq)) markers.push({ board: b, square: sq });
+  }
+  return markers;
+}