feat(runtime): VirtualTagActor skeleton (engine wiring tracked as F8)

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/VirtualTags/VirtualTagActor.cs

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+using Akka.Actor;
+using Akka.Event;
+using ZB.MOM.WW.OtOpcUa.Commons.Types;
+
+namespace ZB.MOM.WW.OtOpcUa.Runtime.VirtualTags;
+
+/// <summary>
+/// Wraps a single virtual-tag expression. Receives dependency-tag updates, recomputes the
+/// expression, and publishes the result to <c>OpcUaPublishActor</c>.
+///
+/// Engine wiring (compile expression via <c>VirtualTagEngine</c>, manage subscriptions,
+/// emit <c>AttributeValueUpdate</c>) is staged for follow-up F8. This skeleton compiles + has
+/// a basic message contract so DriverHostActor can spawn it as a child.
+/// </summary>
+public sealed class VirtualTagActor : ReceiveActor
+{
+    public sealed record DependencyValueChanged(string TagId, object? Value, DateTime TimestampUtc);
+    public sealed record EvaluationResult(string VirtualTagId, object? Value, DateTime TimestampUtc, CorrelationId Correlation);
+
+    private readonly string _virtualTagId;
+    private readonly string _expression;
+    private readonly ILoggingAdapter _log = Context.GetLogger();
+    private readonly Dictionary<string, object?> _dependencies = new(StringComparer.Ordinal);
+
+    public static Props Props(string virtualTagId, string expression) =>
+        Akka.Actor.Props.Create(() => new VirtualTagActor(virtualTagId, expression));
+
+    public VirtualTagActor(string virtualTagId, string expression)
+    {
+        _virtualTagId = virtualTagId;
+        _expression = expression;
+
+        Receive<DependencyValueChanged>(msg =>
+        {
+            _dependencies[msg.TagId] = msg.Value;
+            // Engine wiring (F8): VirtualTagEngine.Evaluate(_expression, _dependencies) → publish.
+            _log.Debug("VirtualTag {Id}: dependency {Tag}={Value} buffered (eval staged for F8)",
+                _virtualTagId, msg.TagId, msg.Value);
+        });
+    }
+}

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/VirtualTags/VirtualTagActorTests.cs

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+using Akka.Actor;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Runtime.Tests.Harness;
+using ZB.MOM.WW.OtOpcUa.Runtime.VirtualTags;
+
+namespace ZB.MOM.WW.OtOpcUa.Runtime.Tests.VirtualTags;
+
+public sealed class VirtualTagActorTests : RuntimeActorTestBase
+{
+    [Fact]
+    public void DependencyValueChanged_is_accepted_and_actor_stays_alive()
+    {
+        var actor = Sys.ActorOf(VirtualTagActor.Props("vt-1", "a + b"));
+        Watch(actor);
+        actor.Tell(new VirtualTagActor.DependencyValueChanged("tag-a", 10, DateTime.UtcNow));
+        actor.Tell(new VirtualTagActor.DependencyValueChanged("tag-b", 20, DateTime.UtcNow));
+
+        // No crash, no termination.
+        ExpectNoMsg(TimeSpan.FromMilliseconds(200));
+    }
+}