docs(dcl): document primary/backup endpoint redundancy across requirements and test infra

docs/requirements/HighLevelReqs.md

@@ -65,6 +65,7 @@
 - Additional protocols can be added by implementing the common interface.
 - The Data Connection Layer is a **clean data pipe** — it publishes tag value updates to Instance Actors but performs no evaluation of triggers or alarm conditions.
 - **Initial attribute quality**: Attributes bound to a data connection start with **uncertain** quality when the Instance Actor initializes. The quality remains uncertain until the first value update is received from the Data Connection Layer. This distinguishes "never received a value" from "received a known-good value" or "connection lost" (bad quality).
+- Data connections support optional **backup endpoints** with automatic failover after a configurable retry count. On failover, all subscriptions are transparently re-created on the new endpoint.
 
 ### 2.5 Scale
 - Approximately **10 sites**.