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.

tests/ScadaLink.SiteRuntime.Tests/Actors/InstanceActorTests.cs

@@ -222,4 +222,61 @@ public class InstanceActorTests : TestKit, IDisposable
         var response = ExpectMsg<GetAttributeResponse>();
         Assert.Equal("98.6", response.Value?.ToString());
     }
+
+    [Fact]
+    public void InstanceActor_DataSourcedAttribute_StartsWithUncertainQuality()
+    {
+        var config = new FlattenedConfiguration
+        {
+            InstanceUniqueName = "Pump1",
+            Attributes =
+            [
+                new ResolvedAttribute
+                {
+                    CanonicalName = "Temperature",
+                    Value = "0",
+                    DataType = "Double",
+                    DataSourceReference = "/Motor/Temperature",
+                    BoundDataConnectionName = "OpcServer1"
+                }
+            ]
+        };
+
+        var actor = CreateInstanceActor("Pump1", config);
+
+        actor.Tell(new GetAttributeRequest(
+            "corr-quality-1", "Pump1", "Temperature", DateTimeOffset.UtcNow));
+
+        var response = ExpectMsg<GetAttributeResponse>();
+        Assert.True(response.Found);
+        Assert.Equal("Uncertain", response.Quality);
+    }
+
+    [Fact]
+    public void InstanceActor_StaticAttribute_StartsWithGoodQuality()
+    {
+        var config = new FlattenedConfiguration
+        {
+            InstanceUniqueName = "Pump1",
+            Attributes =
+            [
+                new ResolvedAttribute
+                {
+                    CanonicalName = "Label",
+                    Value = "Main Pump",
+                    DataType = "String"
+                    // No DataSourceReference — static attribute
+                }
+            ]
+        };
+
+        var actor = CreateInstanceActor("Pump1", config);
+
+        actor.Tell(new GetAttributeRequest(
+            "corr-quality-2", "Pump1", "Label", DateTimeOffset.UtcNow));
+
+        var response = ExpectMsg<GetAttributeResponse>();
+        Assert.True(response.Found);
+        Assert.Equal("Good", response.Quality);
+    }
 }