perf(welcome): index connection tree to rebuild groups in linear time

[desperadoxhy]

问题

WelcomeViewModel.rebuildTree 对每个 group 单独算 connectionCount / depthOf / maxDescendantDepth(各自递归全扫),buildGroupTree 每层 filter 全表,O(G²+C·G),被 18 处连接操作触发。连接/分组多时每次操作卡。

方案

新增 buildGroupTreeIndexed + computeGroupTreeIndices:一次遍历建索引,BFS 算 depth,自底向上聚合 subtree count + maxDepth,降到 O(G+C)。入口重建索引(非增量,无一致性风险)。原 buildGroupTree / connectionCount / depthOf / maxDescendantDepth 公共 API 不变。treeItems 输出结构不变。

验证

• BUILD SUCCEEDED
• 随机属性测试:32 个随机拓扑(1-12 group、1-15 连接,含嵌套/孤儿/dangling 引用)断言新算法与原算法在树结构、depth、maxDepth、count 四项完全等价
• 过程中发现并修复 maxDescendantDepth 漏 1+ 的 bug(属性测试捕获)
• WelcomeViewModelTests 原测试无回归

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[desperadoxhy]

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