fix(runtime): VirtualTagHost watches children + respawns after unexpected death

Context.Watch each spawned child; OnChildTerminated evicts it from _children so the
next ApplyVirtualTags (still containing that vtagId) falls through the ContainsKey
guard and re-spawns a fresh VirtualTagActor.  Adds a spawn-site Debug log, moves the
TODO about in-place plan mutation to the skip-existing branch where it belongs, and
adds a deterministic TestKit test (Child_is_respawned_after_unexpected_termination)
that kills the first child, drains its UnregisterInterest from the mux probe, re-applies,
and asserts a second distinct RegisterInterest arrives.

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

@@ -64,6 +64,7 @@ public sealed class VirtualTagHostActor : ReceiveActor
 
         Receive<ApplyVirtualTags>(OnApply);
         Receive<VirtualTagActor.EvaluationResult>(OnResult);
+        Receive<Terminated>(OnChildTerminated);
     }
 
     private void OnApply(ApplyVirtualTags msg)
@@ -93,6 +94,9 @@ public sealed class VirtualTagHostActor : ReceiveActor
         // a child whose plan changed (the diff already identifies ChangedEquipmentVirtualTags).
         foreach (var p in msg.Plans)
         {
+            // TODO(equipment-virtualtags): when a plan's Expression/DependencyRefs change in place
+            // (ChangedEquipmentVirtualTags), stop+respawn the child here; today only spawn-new/stop-removed
+            // is handled (loader vtags are stable).
             if (_children.ContainsKey(p.VirtualTagId)) continue;
 
             // Auto-name the child: vtagIds can contain characters illegal in actor names, so let Akka
@@ -105,7 +109,9 @@ public sealed class VirtualTagHostActor : ReceiveActor
                 publisherFactory: null,
                 dependencyRefs: p.DependencyRefs,
                 mux: _mux));
+            Context.Watch(child);
             _children[p.VirtualTagId] = child;
+            _log.Debug("VirtualTagHost: spawned child for vtag {VirtualTagId}", p.VirtualTagId);
         }
 
         _log.Debug("VirtualTagHost: applied (desired={Desired}, children={Children})",
@@ -125,6 +131,18 @@ public sealed class VirtualTagHostActor : ReceiveActor
             nodeId, result.Value, OpcUaQuality.Good, result.TimestampUtc));
     }
 
+    private void OnChildTerminated(Terminated msg)
+    {
+        var stale = _children.Where(kv => kv.Value.Equals(msg.ActorRef)).Select(kv => kv.Key).ToList();
+        foreach (var id in stale)
+        {
+            _children.Remove(id);
+            // NodeId map is rebuilt on the next ApplyVirtualTags; leaving the mapping is harmless
+            // (no child will publish for it until respawned). A dead child is respawned on next apply.
+            _log.Warning("VirtualTagHost: child for vtag {VirtualTagId} terminated; will respawn on next apply", id);
+        }
+    }
+
     /// <summary>Folder-scoped NodeId for a VirtualTag plan — MUST match
     /// <c>Phase7Applier.MaterialiseEquipmentVirtualTags</c> exactly, or the published value lands on a
     /// NodeId that was never materialised.</summary>