[REQ-085] AlphaHome formal route surface

[jinjunnn]

Goal

将 AlphaHome 从 Portal takeover 迁为正式 home leaf surface。

Parent requirement

jinjunnn/alpha-code#200

Scope

• Deliver the jinjunnn/alpha-code boundary for Alpha REQ-085: AlphaHome formal route surface.
• Preserve the parent requirement contract; do not absorb adjacent repository work.

Out of scope

• Work owned by sibling sub-issues or future REQs.

Acceptance criteria

• 仅一个 Home root、legacy reload 回退与首条消息导航保持。
• Relevant automated and packaged/runtime checks are attached before closure.

Dependencies

REQ-084 verified。

Verification evidence

• CI/test evidence.
• Security/runtime/product acceptance evidence appropriate to this repository.

[jinjunnn]

验收证据(S40,PR #238 已合并)

• ✅ home=alpha:DOM 单一 Home page root(探针 alphaHomeRoots:1),upstream Home lazy 不挂载;AlphaHome 内已无 Portal/isHome/pathname 判断(源码 diff + 视觉验收 01-home-alpha.png)
• ✅ home=legacy:env 覆盖重启 → Alpha 叶不挂载(探针 alphaHomeRoots:0),URL/数据不变,app 正常(03-home-legacy.png);auto-fallback 崩溃回退链路:SurfaceBoundary 记录 surface id/version → 上报 main 落盘 → 确认 reload 后按 appVersion 解析为 legacy(main 解析器 14 项测试)
• ✅ 项目/默认 ~/Alpha/composer/首条消息导航行为不变(AlphaComposer 与 startChat 通路未动,导航走 ABI helper)
• ✅ 首页 Portal/固定 CSS 覆盖删除(a-home--page 成流布局)
• 📌 按用户指令本轮不做冒烟测试,以视觉验收 + DOM 探针替代(docs/audits/2026-07-12-req084-086-visual/);a11y 自动化断言与 takeover CI ratchet 未落地(仓内无 ratchet 设施),残项随 REQ-088/090 批次补