fix(site-runtime): fan tag updates out to every attribute sharing a tag path

InstanceActor._tagPathToAttribute was a Dictionary<string,string> — one tag
path mapped to a single attribute. When two attributes reference the same PLC
node (e.g. two composed cooling-tank modules both reading ns=3;s=Tank.Level,
or a pump's TempSensor and AlarmSensor both reading ns=3;s=Sensor.Reading),
SubscribeToDcl's map assignment overwrote, so only the last-registered
attribute ever received values — the rest stayed permanently Uncertain.

The map is now Dictionary<string,List<string>>; HandleTagValueUpdate fans each
update out to every attribute referencing the tag path, and each distinct tag
path is still subscribed only once per connection.

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

@@ -2,6 +2,8 @@ using Akka.Actor;
 using Akka.TestKit.Xunit2;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Logging.Abstractions;
+using ScadaLink.Commons.Interfaces.Protocol;
+using ScadaLink.Commons.Messages.DataConnection;
 using ScadaLink.Commons.Messages.Instance;
 using ScadaLink.Commons.Messages.Lifecycle;
 using ScadaLink.Commons.Types.Flattening;
@@ -306,4 +308,63 @@ public class InstanceActorTests : TestKit, IDisposable
         Assert.True(response.Found);
         Assert.Equal("Good", response.Quality);
     }
+
+    /// <summary>
+    /// A single PLC tag can back more than one attribute — e.g. two composed
+    /// cooling-tank modules whose members both reference the one simulated
+    /// <c>ns=3;s=Tank.Level</c> node. A <see cref="TagValueUpdate"/> must fan out
+    /// to every attribute that references that tag path, not just the last one
+    /// registered: the tag-path → attribute map previously overwrote on a shared
+    /// tag, leaving all but one attribute permanently Uncertain.
+    /// </summary>
+    [Fact]
+    public void InstanceActor_TagUpdate_FansOutToEveryAttributeSharingTheTagPath()
+    {
+        const string sharedTag = "ns=3;s=Tank.Level";
+        var config = new FlattenedConfiguration
+        {
+            InstanceUniqueName = "Motor-1",
+            Attributes =
+            [
+                new ResolvedAttribute
+                {
+                    CanonicalName = "CoolingTank.Level", Value = "0", DataType = "Int",
+                    DataSourceReference = sharedTag, BoundDataConnectionName = "PLC"
+                },
+                new ResolvedAttribute
+                {
+                    CanonicalName = "CoolingTank2.Level", Value = "0", DataType = "Int",
+                    DataSourceReference = sharedTag, BoundDataConnectionName = "PLC"
+                }
+            ]
+        };
+
+        var dcl = CreateTestProbe();
+        var actor = ActorOf(Props.Create(() => new InstanceActor(
+            "Motor-1",
+            JsonSerializer.Serialize(config),
+            _storage,
+            _compilationService,
+            _sharedScriptLibrary,
+            null,
+            _options,
+            NullLogger<InstanceActor>.Instance,
+            dcl.Ref)));
+
+        // On startup the actor subscribes its data-sourced tags through the DCL.
+        dcl.ExpectMsg<SubscribeTagsRequest>(TimeSpan.FromSeconds(5));
+
+        // One value arrives for the tag that both attributes reference.
+        actor.Tell(new TagValueUpdate("PLC", sharedTag, 47, QualityCode.Good, DateTimeOffset.UtcNow));
+
+        // BOTH attributes must reflect it — not just the last-registered one.
+        foreach (var attrName in new[] { "CoolingTank.Level", "CoolingTank2.Level" })
+        {
+            actor.Tell(new GetAttributeRequest("corr-fanout", "Motor-1", attrName, DateTimeOffset.UtcNow));
+            var response = ExpectMsg<GetAttributeResponse>(TimeSpan.FromSeconds(5));
+            Assert.True(response.Found);
+            Assert.Equal("47", response.Value?.ToString());
+            Assert.Equal("Good", response.Quality);
+        }
+    }
 }