"Multi-Dc"
- MongoDB Shard Expansion + Multi-DC：Type F「不需要 parallel run」的 multi-region 例外
MongoDB sharded cluster 加 shard + 跨 DC expansion 是 Type F「topology re-layout」第 3 個 dogfood — 同時改 sharding + replication topology + region distribution；驗證 [#128](/report/data-topology-as-audit-dimension/) self-aware limitation 第 3 點「Type F 不需要 parallel run」claim 的例外（multi-region rollout 必須 parallel run + 切流量）；涵蓋 chunk migration / replica set add member / cross-DC routing