docs: M3 第二波收口——报告 15~20 入档挂导航

- 15~17 社区后端纵切三单（帖子/Feed 作者链路/评论互动关注，226→310）
- 18 契约冻结 v1.3.0（31 路径/43 操作，26 项修正照单全收）
- 19 快照同步与全仓契约矩阵（173 格零漂移，修 allOf 校验盲区，→325）
- 20 收口总表：定型语义汇总（第三波接入依据）与质量事件记录

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

mkdocs.yml

@@ -78,6 +78,12 @@ nav:
           - 12 防泄漏检查落地: development/iterations/iteration-3/12-secret-scan-rollout.md
           - 13 media MinIO 闭环: development/iterations/iteration-3/13-media-minio-report.md
           - 14 第一波收口: development/iterations/iteration-3/14-wave1-closure.md
+          - 15 帖子生命周期: development/iterations/iteration-3/15-post-lifecycle-report.md
+          - 16 Feed 与作者链路: development/iterations/iteration-3/16-feed-author-report.md
+          - 17 评论与互动: development/iterations/iteration-3/17-comments-interactions-report.md
+          - 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
   - API:
       - 契约说明: api/index.md
   - 架构: