docs: M3 第三波收口——报告 21~27 入档挂导航

- 21~26 Flutter 社区接入五单 + 字典 v3 白名单（flutter 286→502、api 325→334）
- 27 收口总表：社区 demo 三页消亡、M3 四条验收标准逐条取证、
  乐观更新与媒体链路端到端、实现期修正记录
- device-verification.md 的 M3 四项真机步骤已由各单收口补全

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

mkdocs.yml

@@ -84,6 +84,13 @@ nav:
           - 18 契约冻结 v1.3.0: development/iterations/iteration-3/18-contract-freeze-report.md
           - 19 快照同步与矩阵: development/iterations/iteration-3/19-contract-sync-report.md
           - 20 第二波收口: development/iterations/iteration-3/20-wave2-closure.md
+          - 21 社区数据层: development/iterations/iteration-3/21-community-datalayer.md
+          - 22 埋点白名单 v3: development/iterations/iteration-3/22-event-whitelist-v3.md
+          - 23 媒体上传客户端: development/iterations/iteration-3/23-media-upload-client.md
+          - 24 Feed 页接入: development/iterations/iteration-3/24-feed-page-report.md
+          - 25 详情页与互动: development/iterations/iteration-3/25-detail-interactions-report.md
+          - 26 发布页与漏斗: development/iterations/iteration-3/26-publish-page-report.md
+          - 27 第三波收口: development/iterations/iteration-3/27-wave3-closure.md
   - API:
       - 契约说明: api/index.md
   - 架构: