docs: T2-19 文档收口（E2E 前半）——迭代二看板 + 功能清单 M2 增补

- iteration-2/index.md 进展看板：三波交付纪年、测试与契约演进表、遗留清单
- feature-checklist 去掉「第一迭代」限定，新增第 7~9 节（宠物域后端 13 条/
  客户端 10 条/埋点体系 7 条），状态以 dev + 门禁全绿为准
- 待 E2E 收官后补：T2-18 证据归档与 M2 收官总结报告

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

mkdocs.yml

@@ -31,6 +31,7 @@ nav:
           - 19 埋点系统实现报告: development/iterations/iteration-1/19-analytics-implementation-report.md
           - 20 第一迭代收官总结: development/iterations/iteration-1/20-iteration-1-summary.md
       - 第二迭代:
+          - 进展看板: development/iterations/iteration-2/index.md
           - 01 任务分解: development/iterations/iteration-2/01-pm-task-breakdown.md
           - 02 后端技术评估: development/iterations/iteration-2/02-backend-technical-assessment.md
           - 03 Flutter 技术评估: development/iterations/iteration-2/03-flutter-technical-assessment.md