fix(virtual-tags): resolve High code-review finding (Core.VirtualTags-001)

OnScriptSetVirtualTag updated the value cache, notified observers, and
recorded history for the written path but never scheduled a cascade for
tags depending on that path. This contradicts docs/VirtualTags.md, which
states ctx.SetVirtualTag writes "still participate in change-trigger
cascades": a change-triggered virtual tag reading a script-written tag
went stale until an unrelated trigger fired.

OnScriptSetVirtualTag now launches a fire-and-forget CascadeAsync for the
written path, mirroring OnUpstreamChange. The cascade is scheduled rather
than invoked inline because the callback runs inside EvaluateInternalAsync
while the non-reentrant _evalGate semaphore is held.

Added regression test
SetVirtualTag_within_script_cascades_to_dependents_of_the_written_tag.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Core/ZB.MOM.WW.OtOpcUa.Core.VirtualTags/VirtualTagEngine.cs

@@ -315,6 +315,14 @@ public sealed class VirtualTagEngine : IDisposable
         _valueCache[path] = snap;
         NotifyObservers(path, snap);
         if (_tags[path].Definition.Historize) _history.Record(path, snap);
+
+        // A cross-tag write must participate in the change-trigger cascade, exactly
+        // like an upstream delta — any change-triggered tag that reads this path
+        // would otherwise go stale until an unrelated trigger fires (see
+        // docs/VirtualTags.md, VirtualTagContext section). Fire-and-forget: this
+        // callback runs inside EvaluateInternalAsync with the non-reentrant
+        // _evalGate held, so the cascade must be scheduled, not invoked inline.
+        _ = CascadeAsync(path, CancellationToken.None);
     }
 
     private void NotifyObservers(string path, DataValueSnapshot value)