docs: establish green baseline for phase two routing

[WilliamWang1721]

目标

先建立第二阶段的全绿基线，再逐步迁移协议路由。

当前变更

• 仅新增第二阶段路由迁移架构文档
• 不改动生产路由代码
• 不引入任何自动写回工作流

合并条件

• CI 全部通过
• Security Scan 全部通过
• 前端、单测、集成测试、lint 全绿

基线全绿后，再按“一个端点族一个提交”的方式继续第二阶段开发。

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