docs: initialize rewrite plan and freeze backend surfaces

backend-rewrite-tasklist/02_M3_RUNTIME_PROFILE.md

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+# M3：runtime snapshot / settings / profile 任务清单
+
+## 1. SpacetimeDB 运行时主表
+
+- [ ] 设计 `runtime_snapshot`
+- [ ] 设计 `runtime_setting`
+- [ ] 设计 `profile_dashboard_state`
+- [ ] 设计 `profile_wallet_ledger`
+- [ ] 设计 `profile_played_world`
+- [ ] 设计 `profile_save_archive`
+- [ ] 设计 `user_browse_history`
+
+## 2. 兼容快照策略
+
+- [ ] 设计“领域表真相 + 兼容聚合快照”策略
+- [ ] 设计 snapshot projection 刷新机制
+- [ ] 迁移当前 snapshot hydration / normalize 规则
+- [ ] 迁移当前 save archive 聚合逻辑
+- [ ] 迁移当前 browse history 去重与排序逻辑
+
+## 3. Axum facade
+
+- [ ] 兼容 `GET /api/runtime/save/snapshot`
+- [ ] 兼容 `PUT /api/runtime/save/snapshot`
+- [ ] 兼容 `DELETE /api/runtime/save/snapshot`
+- [ ] 兼容 `GET /api/runtime/settings`
+- [ ] 兼容 `PUT /api/runtime/settings`
+- [ ] 兼容 `GET /api/runtime/profile/dashboard`
+- [ ] 兼容 `GET /api/profile/dashboard`
+- [ ] 兼容 `GET /api/runtime/profile/wallet-ledger`
+- [ ] 兼容 `GET /api/profile/wallet-ledger`
+- [ ] 兼容 `GET /api/runtime/profile/play-stats`
+- [ ] 兼容 `GET /api/profile/play-stats`
+- [ ] 兼容 `GET /api/runtime/profile/save-archives`
+- [ ] 兼容 `GET /api/profile/save-archives`
+- [ ] 兼容 `POST /api/runtime/profile/save-archives/:worldKey`
+- [ ] 兼容 `POST /api/profile/save-archives/:worldKey`
+- [ ] 兼容 `GET /api/runtime/profile/browse-history`
+- [ ] 兼容 `POST /api/runtime/profile/browse-history`
+- [ ] 兼容 `DELETE /api/runtime/profile/browse-history`
+- [ ] 兼容 `GET /api/profile/browse-history`
+- [ ] 兼容 `POST /api/profile/browse-history`
+- [ ] 兼容 `DELETE /api/profile/browse-history`
+
+## 4. 阶段验收
+
+- [ ] 登录用户可正常保存、读取、删除存档
+- [ ] 兼容路径与主路径返回一致
+- [ ] profile dashboard / browse history / save archive 行为一致
+- [ ] 前端当前恢复流程可在不改 UI 的前提下跑通