feat(runtime): #113 DependencyMuxActor — drivers → virtual-tag fan-out

End-to-end data path is now wired on the read side: driver subscriptions
fire AttributeValuePublished → DriverHostActor → DependencyMuxActor →
DependencyValueChanged to every interested VirtualTagActor. Previously
the publish hit a dead-letter at the host.

DependencyMuxActor:
  - Per-node fan-out router. Maintains tagRef → Set<IActorRef> with a
    reverse subscriber → refs index so unregister/replace are O(refs).
  - Watches subscribers; Terminated triggers automatic unregister so
    dead virtual-tag actors stop receiving publishes.
  - Re-register replaces the prior interest set — no stale-ref leaks
    on actor restart.
  - Drops publishes for refs with no interested subscribers.

VirtualTagActor:
  - New Props params: dependencyRefs + mux ActorRef.
  - PreStart sends RegisterInterest to the mux; PostStop sends
    UnregisterInterest. Default both null so older callers stay quiet.

DriverHostActor:
  - New dependencyMux Props param. Steady + Applying states now
    receive AttributeValuePublished from their DriverInstance children
    and forward to the mux. Null mux is a no-op (dev/Mac).

ServiceCollectionExtensions:
  - WithOtOpcUaRuntimeActors spawns DependencyMuxActor before
    DriverHostActor and threads its ActorRef into the host's Props.
    New DependencyMuxActorKey + DependencyMuxActorName.

Tests: Runtime 57 -> 63 (+6):
- Mux forwards to only subscribers interested in each ref
- Publish for unregistered ref is dropped silently
- Unregister stops forwarding
- Re-register replaces prior interest set
- VirtualTagActor PreStart registration drives end-to-end eval
  (uses AwaitAssert to race-safely settle the PreStart Tell)
- DriverHostActor forwards AttributeValuePublished through to mux

All 6 v2 test suites green: 163 tests passing.

F8 (#79) state updated — dep subscribe seam shipped, Core.VirtualTags
production engine binding (compile + ITagUpstreamSource subscribe) is
the residual.

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/ServiceCollectionExtensions.cs

@@ -10,6 +10,7 @@ using ZB.MOM.WW.OtOpcUa.Core.AlarmHistorian;
 using ZB.MOM.WW.OtOpcUa.Runtime.Drivers;
 using ZB.MOM.WW.OtOpcUa.Runtime.Health;
 using ZB.MOM.WW.OtOpcUa.Runtime.Historian;
+using ZB.MOM.WW.OtOpcUa.Runtime.VirtualTags;
 
 namespace ZB.MOM.WW.OtOpcUa.Runtime;
 
@@ -20,6 +21,7 @@ public static class ServiceCollectionExtensions
     public const string DriverHostActorName = "driver-host";
     public const string DbHealthProbeActorName = "db-health";
     public const string HistorianAdapterActorName = "historian-adapter";
+    public const string DependencyMuxActorName = "dependency-mux";
 
     /// <summary>
     /// Registers shared runtime services. Currently binds <see cref="IAlarmHistorianSink"/>
@@ -65,9 +67,15 @@ public static class ServiceCollectionExtensions
                 DbHealthProbeActorName);
             registry.Register<DbHealthProbeActorKey>(dbHealth);
 
+            // Dependency mux must be spawned before DriverHostActor so the host can forward
+            // AttributeValuePublished into it from the very first driver spawn.
+            var mux = system.ActorOf(DependencyMuxActor.Props(), DependencyMuxActorName);
+            registry.Register<DependencyMuxActorKey>(mux);
+
             var driverHost = system.ActorOf(
                 DriverHostActor.Props(dbFactory, roleInfo.LocalNode, coordinator: null,
-                    driverFactory: driverFactory, localRoles: roleInfo.LocalRoles),
+                    driverFactory: driverFactory, localRoles: roleInfo.LocalRoles,
+                    dependencyMux: mux),
                 DriverHostActorName);
             registry.Register<DriverHostActorKey>(driverHost);
 
@@ -85,3 +93,4 @@ public static class ServiceCollectionExtensions
 public sealed class DriverHostActorKey { }
 public sealed class DbHealthProbeActorKey { }
 public sealed class HistorianAdapterActorKey { }
+public sealed class DependencyMuxActorKey { }