nav: 三级导航分页结构 + 中国地区索引

- 02-game-design 地图结构重写：洲→国家→城市，3x3棋盘分页
- 中国 40 城按 8 地区分组（华北/东北/华东/华中/华南/西南/西北/港澳）
- 导航索引数据：continents/index + asia + regions/china + pagination工具

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

js/content/shared/level-presets.js

@@ -0,0 +1,70 @@
+export function createLevelPresets(seedBase) {
+  return [
+    {
+      id: 1,
+      difficultyTier: 'intro',
+      seedBase: seedBase + 1,
+      elementCount: 6,
+      piecesPerElement: 3,
+      layers: 2,
+      density: 'low',
+      targetPassRate: 0.95,
+      targetDurationSec: [90, 150],
+    },
+    {
+      id: 2,
+      difficultyTier: 'warmup',
+      seedBase: seedBase + 2,
+      elementCount: 7,
+      piecesPerElement: 3,
+      layers: 2,
+      density: 'low',
+      targetPassRate: 0.9,
+      targetDurationSec: [100, 160],
+    },
+    {
+      id: 3,
+      difficultyTier: 'ramp',
+      seedBase: seedBase + 3,
+      elementCount: 8,
+      piecesPerElement: 3,
+      layers: 3,
+      density: 'medium',
+      targetPassRate: 0.8,
+      targetDurationSec: [120, 180],
+    },
+    {
+      id: 4,
+      difficultyTier: 'mid',
+      seedBase: seedBase + 4,
+      elementCount: 9,
+      piecesPerElement: 3,
+      layers: 3,
+      density: 'medium',
+      targetPassRate: 0.7,
+      targetDurationSec: [130, 190],
+    },
+    {
+      id: 5,
+      difficultyTier: 'hard',
+      seedBase: seedBase + 5,
+      elementCount: 10,
+      piecesPerElement: 3,
+      layers: 4,
+      density: 'medium_high',
+      targetPassRate: 0.6,
+      targetDurationSec: [150, 220],
+    },
+    {
+      id: 6,
+      difficultyTier: 'boss',
+      seedBase: seedBase + 6,
+      elementCount: 10,
+      piecesPerElement: [3, 3, 3, 3, 3, 6, 3, 3, 3, 3],
+      layers: 4,
+      density: 'high',
+      targetPassRate: 0.55,
+      targetDurationSec: [180, 260],
+    },
+  ]
+}

js/content/shared/pagination.js

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+// 通用分页工具 — 将数组按 pageSize 分页
+// 默认 pageSize = 9（3x3 棋盘）
+
+export function paginate(items, pageSize = 9) {
+  const pages = []
+  for (let i = 0; i < items.length; i += pageSize) {
+    pages.push(items.slice(i, i + pageSize))
+  }
+  return pages
+}
+
+export function getPageCount(totalItems, pageSize = 9) {
+  return Math.ceil(totalItems / pageSize)
+}