feat: data-sourced attributes start with uncertain quality before first DCL value

Attributes bound to data connections now initialize with "Uncertain" quality,
distinguishing "never received a value" from "known good" or "connection lost."
Quality is tracked per attribute and included in GetAttributeResponse.

HighLevelReqs.md

@@ -60,6 +60,7 @@
 - Both protocols implement a **common interface** supporting: connect, subscribe to tag paths, receive value updates, and write values.
 - 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).
 
 ### 2.5 Scale
 - Approximately **10 sites**.