refactor(chat): 把 MessageList 的行为引擎抽成模块内 hook - #531
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MessageList 838 行里内联着三套彼此独立的引擎逻辑，业务接线被淹在里面。
参照「引擎与壳分离」的形状（web 端引擎在包里、RN 生态没有现成等价物，
LegendList/RNKC 都是打了 patch 才能用的，只能引擎进 hook）拆成三个模块私有
hook，MessageList 缩成 318 行接线层：
- useTailFollow：尾随状态机（anchoring→following→paused）+ 全部交互状态。
isUserInteractingRef 由它单一持有、对外只读——ready-gate 等其他自动滚动源
必须与它守同一个「用户手上有动作时一律不滚」的不变式。
- useAnchorPin：钉顶（handleAnchorReady）、单轮工作区首锚 staging、ready-gate
静默窗口。预留空白耗尽时经 notifyAnchorSpaceClosed 把相位交接给尾随，
避免两个 hook 双写同一状态。
- useLayoutBenchInstrumentation：探针常驻副作用（UI 线程回抛、键盘时间线、
armed 同步）。信息发生在业务回调体内的调用点（anchorReady/readyGate 的
progScroll、endSpace 上报）保留裸函数形式留在现场，搬不走。
行为保持重构：传给 LegendList 的 props 逐值不变，单测断言零改动全绿
（10 套件 59 测试）；被实测纠正过的长注释逐字随代码迁移。跨域组合
（handleItemSizeChanged 同时触发首锚释放与尾随）留在组件当接线。
bench 前后各三轮零违规，关键指标同带：
follow-up-turn offset-reversal 17/17/24 → 24/24/24（阈值 100）
estimate-collapse 252 与 5 前后逐轮不变
send/stream offset-reversal 0-64 区间内波动，与文档确定性表的
历史噪声带逐档吻合
「改了没变化」用探针版本标记做了阳性对照（v:2 临时字段在设备 trace 出现后
撤除），排除 Fast Refresh/字节码缓存假阴性。
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行为保持重构：MessageList 838 行里内联着三套彼此独立的引擎逻辑，业务接线被淹在里面。拆成三个模块私有 hook（
components/hooks/，不出模块 index），MessageList 缩成 318 行接线层。useTailFollow（274 行）isUserInteractingRef单一持有、对外只读——ready-gate 等其他自动滚动源必须与它守同一个「用户手上有动作时一律不滚」的不变式useAnchorPin（297 行）notifyAnchorSpaceClosed把相位交接给尾随，两个 hook 不双写同一状态useLayoutBenchInstrumentation（140 行）跨域组合（
handleItemSizeChanged同时触发首锚释放与尾随）留在组件当接线。被实测纠正过的长注释逐字随代码迁移。验证
传给 LegendList 的 props 逐值不变；单测断言零改动全绿（messagePresentation 10 套件 59 测试；全量 310 套件 2896 测试）。
bench 前后各三轮零违规（feat(chat-bench): 聊天布局基准 harness——场景、相位感知判据与文档 #530 的 harness 就是本次重构的回归护栏，这是它第一次实战）：
「改了没变化」做了阳性对照：临时给 phase 探针加
v:2字段、确认出现在设备 trace 后撤除，排除 Fast Refresh/字节码缓存假阴性（此前有两次因此得出错误结论的记录）。🤖 Generated with Claude Code