feat: add gift zone city team and mashup mode

tests/difficulty-generator.test.js

@@ -6,6 +6,7 @@ import {
   classifyDeadlock,
   evaluateBoard,
   generateBoard,
+  generateMashupBoard,
 } from '../js/gameplay/difficulty/index.js'
 
 test('generateBoard is deterministic for the same city, level, and seed', () => {
@@ -72,3 +73,23 @@ test('classifyDeadlock identifies a hard deadlock when slot is full and no match
 
   assert.equal(result.type, 'hard')
 })
+
+test('generateMashupBoard is deterministic and mixes unlocked city content', () => {
+  const contentSystem = createContentSystem()
+
+  const first = generateMashupBoard({
+    cityIds: ['beijing', 'tokyo'],
+    seed: 33001,
+    contentSystem,
+  })
+  const second = generateMashupBoard({
+    cityIds: ['beijing', 'tokyo'],
+    seed: 33001,
+    contentSystem,
+  })
+
+  assert.deepEqual(first.pieces, second.pieces)
+  assert.deepEqual(first.overlapGraph, second.overlapGraph)
+  assert.deepEqual(first.sourceCityIds, ['beijing', 'tokyo'])
+  assert.ok(new Set(first.elementDefinitions.map((entry) => entry.sourceCityId)).size >= 2)
+})