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/Drivers/DriverHostActor.cs

@@ -41,6 +41,7 @@ public sealed class DriverHostActor : ReceiveActor, IWithTimers
     private readonly IActorRef? _coordinatorOverride;
     private readonly IDriverFactory _driverFactory;
     private readonly IReadOnlySet<string> _localRoles;
+    private readonly IActorRef? _dependencyMux;
     private readonly ILoggingAdapter _log = Context.GetLogger();
 
     private RevisionHash? _currentRevision;
@@ -63,21 +64,25 @@ public sealed class DriverHostActor : ReceiveActor, IWithTimers
         CommonsNodeId localNode,
         IActorRef? coordinator = null,
         IDriverFactory? driverFactory = null,
-        IReadOnlySet<string>? localRoles = null) =>
-        Akka.Actor.Props.Create(() => new DriverHostActor(dbFactory, localNode, coordinator, driverFactory, localRoles));
+        IReadOnlySet<string>? localRoles = null,
+        IActorRef? dependencyMux = null) =>
+        Akka.Actor.Props.Create(() => new DriverHostActor(
+            dbFactory, localNode, coordinator, driverFactory, localRoles, dependencyMux));
 
     public DriverHostActor(
         IDbContextFactory<OtOpcUaConfigDbContext> dbFactory,
         CommonsNodeId localNode,
         IActorRef? coordinator,
         IDriverFactory? driverFactory = null,
-        IReadOnlySet<string>? localRoles = null)
+        IReadOnlySet<string>? localRoles = null,
+        IActorRef? dependencyMux = null)
     {
         _dbFactory = dbFactory;
         _localNode = localNode;
         _coordinatorOverride = coordinator;
         _driverFactory = driverFactory ?? NullDriverFactory.Instance;
         _localRoles = localRoles ?? new HashSet<string>(StringComparer.Ordinal);
+        _dependencyMux = dependencyMux;
 
         // Default behavior is Steady — PreStart may flip to Stale or replay an orphan apply.
         Become(Steady);
@@ -150,6 +155,7 @@ public sealed class DriverHostActor : ReceiveActor, IWithTimers
     {
         Receive<DispatchDeployment>(HandleDispatchFromSteady);
         Receive<GetDiagnostics>(HandleGetDiagnostics);
+        Receive<DriverInstanceActor.AttributeValuePublished>(ForwardToMux);
         Receive<SubscribeAck>(_ => { /* PubSub ack */ });
     }
 
@@ -168,9 +174,18 @@ public sealed class DriverHostActor : ReceiveActor, IWithTimers
             Self.Forward(msg); // re-deliver after we transition back
         });
         Receive<GetDiagnostics>(HandleGetDiagnostics);
+        Receive<DriverInstanceActor.AttributeValuePublished>(ForwardToMux);
         Receive<SubscribeAck>(_ => { /* PubSub ack */ });
     }
 
+    private void ForwardToMux(DriverInstanceActor.AttributeValuePublished msg)
+    {
+        // Pass driver-published values to the dependency mux when one is wired. Without a mux,
+        // VirtualTagActor evaluation can't fire — values just drop here. That's the dev/Mac path
+        // (no virtual tags registered); production binds the mux via the RuntimeActors extension.
+        _dependencyMux?.Tell(msg);
+    }
+
     private void Stale()
     {
         Receive<DispatchDeployment>(_ =>