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.

src/ScadaLink.Commons/Messages/Instance/GetAttributeRequest.cs

@@ -11,7 +11,9 @@ public record GetAttributeRequest(
     DateTimeOffset Timestamp);
 
 /// <summary>
-/// Response containing the current value of an attribute.
+/// Response containing the current value and quality of an attribute.
+/// Quality is "Good", "Bad", or "Uncertain".
+/// Data-sourced attributes start at "Uncertain" until the first DCL value update arrives.
 /// </summary>
 public record GetAttributeResponse(
     string CorrelationId,
@@ -19,4 +21,5 @@ public record GetAttributeResponse(
     string AttributeName,
     object? Value,
     bool Found,
+    string Quality,
     DateTimeOffset Timestamp);