feat: add gift zone city team and mashup mode

js/gameplay/difficulty/index.js

@@ -1,6 +1,7 @@
 export { classifyDeadlock } from './classify-deadlock.js'
 export { evaluateBoard } from './evaluate-board.js'
 export { generateBoard } from './generate-board.js'
+export { generateMashupBoard } from './generate-mashup-board.js'
 export {
   buildOverlapGraph,
   getClickablePieces,