docs: M3 开工分析 8 份报告入档 + ADR-016~021 拍板决策

- iteration-3 报告 01-08（PM 拆解/后端/Flutter/RC 首个 CERTIFIED/UI/埋点/证据基线/Git）
- ADR-016 自托管 MinIO 起步预留迁云（用户确认现无云存储）、ADR-017 patbond-community
  :8084 + media 归 user + 作者信息跨 schema 只读、ADR-018 范围裁剪（话题剪出/单层评论）、
  ADR-019 幂等按域（PUT/DELETE + request_hash）、ADR-020 聚合 feed_viewed/字典 v3/
  北极星不变/队列三项升第一波、ADR-021 Git 修订（main 更正/PR 情形触发/首发布重建 main/
  防泄漏 grep 先行）
- mkdocs 挂「第三迭代」导航，build --strict 通过

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

mkdocs.yml

@@ -62,6 +62,15 @@ nav:
           - 28 E2E 烟囱收官: development/iterations/iteration-2/28-e2e-smoke-report.md
           - 29 M2 收官总结: development/iterations/iteration-2/29-m2-summary.md
           - 30 真机补验清单: development/iterations/iteration-2/30-device-verification-checklist.md
+      - 第三迭代:
+          - 01 任务分解: development/iterations/iteration-3/01-pm-task-breakdown.md
+          - 02 后端技术评估: development/iterations/iteration-3/02-backend-technical-assessment.md
+          - 03 Flutter 技术评估: development/iterations/iteration-3/03-flutter-technical-assessment.md
+          - 04 现状核实: development/iterations/iteration-3/04-reality-check.md
+          - 05 社区 UI 设计规范: development/iterations/iteration-3/05-community-ui-spec.md
+          - 06 埋点规划: development/iterations/iteration-3/06-experiment-tracking-plan.md
+          - 07 证据基线审计: development/iterations/iteration-3/07-evidence-baseline-audit.md
+          - 08 Git 工作流规划: development/iterations/iteration-3/08-git-workflow-plan.md
   - API:
       - 契约说明: api/index.md
   - 架构: