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);

src/ScadaLink.SiteRuntime/Actors/InstanceActor.cs

@@ -38,6 +38,7 @@ public class InstanceActor : ReceiveActor
     private readonly SiteRuntimeOptions _options;
     private readonly ILogger _logger;
     private readonly Dictionary<string, object?> _attributes = new();
+    private readonly Dictionary<string, string> _attributeQualities = new();
     private readonly Dictionary<string, AlarmState> _alarmStates = new();
     private readonly Dictionary<string, IActorRef> _scriptActors = new();
     private readonly Dictionary<string, IActorRef> _alarmActors = new();
@@ -75,11 +76,15 @@ public class InstanceActor : ReceiveActor
         _configuration = JsonSerializer.Deserialize<FlattenedConfiguration>(configJson);
 
         // Load default attribute values from the flattened configuration
+        // Data-sourced attributes start with Uncertain quality until the first DCL value arrives.
+        // Static attributes start with Good quality.
         if (_configuration != null)
         {
             foreach (var attr in _configuration.Attributes)
             {
                 _attributes[attr.CanonicalName] = attr.Value;
+                _attributeQualities[attr.CanonicalName] =
+                    string.IsNullOrEmpty(attr.DataSourceReference) ? "Good" : "Uncertain";
             }
         }
 
@@ -169,12 +174,14 @@ public class InstanceActor : ReceiveActor
     private void HandleGetAttribute(GetAttributeRequest request)
     {
         var found = _attributes.TryGetValue(request.AttributeName, out var value);
+        _attributeQualities.TryGetValue(request.AttributeName, out var quality);
         Sender.Tell(new GetAttributeResponse(
             request.CorrelationId,
             _instanceUniqueName,
             request.AttributeName,
             value,
             found,
+            quality ?? "Good",
             DateTimeOffset.UtcNow));
     }
 
@@ -249,6 +256,7 @@ public class InstanceActor : ReceiveActor
     {
         // WP-24: State mutation serialized through this actor
         _attributes[changed.AttributeName] = changed.Value;
+        _attributeQualities[changed.AttributeName] = changed.Quality;
 
         PublishAndNotifyChildren(changed);
     }
@@ -345,7 +353,7 @@ public class InstanceActor : ReceiveActor
             kvp.Key,
             kvp.Key,
             kvp.Value,
-            "Good",
+            _attributeQualities.GetValueOrDefault(kvp.Key, "Good"),
             DateTimeOffset.UtcNow)).ToList();
 
         var alarmStates = _alarmStates.Select(kvp => new AlarmStateChanged(