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>

code-reviews/Core.VirtualTags/findings.md

@@ -7,7 +7,7 @@
 | Review date | 2026-05-22 |
 | Commit reviewed | `76d35d1` |
 | Status | Reviewed |
-| Open findings | 13 |
+| Open findings | 12 |
 
 ## Checklist coverage
 
@@ -36,7 +36,7 @@ a category produced nothing rather than leaving it blank.
 | Severity | High |
 | Category | Correctness & logic bugs |
 | Location | `src/Core/ZB.MOM.WW.OtOpcUa.Core.VirtualTags/VirtualTagEngine.cs:306` |
-| Status | Open |
+| Status | Resolved |
 
 **Description:** `OnScriptSetVirtualTag` updates `_valueCache`, notifies observers, and
 records history for the written path, but it does not schedule a cascade for tags that
@@ -58,7 +58,7 @@ held), or (b) if cascading from a script write is intentionally unsupported, cor
 documentation and `VirtualTagContext` XML doc to say so. Decide deliberately and make
 code and docs agree.
 
-**Resolution:** _(open)_
+**Resolution:** Resolved 2026-05-22 — `OnScriptSetVirtualTag` now launches a fire-and-forget `CascadeAsync(path, ...)` after updating the cache, mirroring `OnUpstreamChange`, so change-triggered dependents of a script-written tag are re-evaluated; added a regression test.
 
 ### Core.VirtualTags-002
 

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)

tests/Core/ZB.MOM.WW.OtOpcUa.Core.VirtualTags.Tests/VirtualTagEngineTests.cs

@@ -275,6 +275,38 @@ public sealed class VirtualTagEngineTests
         engine.Read("Driver").Value.ShouldBe(5);
     }
 
+    [Fact]
+    public async Task SetVirtualTag_within_script_cascades_to_dependents_of_the_written_tag()
+    {
+        // Regression for Core.VirtualTags-001: a ctx.SetVirtualTag write must schedule
+        // the change-trigger cascade for tags depending on the written path, exactly
+        // like an upstream delta. "Dependent" reads "Target" and is ChangeTriggered, so
+        // a write to Target via ctx.SetVirtualTag must re-evaluate Dependent.
+        var up = new FakeUpstream();
+        up.Set("In", 5);
+        using var engine = Build(up);
+
+        engine.Load([
+            new VirtualTagDefinition("Target", DriverDataType.Int32,
+                """return 0;""", ChangeTriggered: false),  // operator-written via SetVirtualTag
+            new VirtualTagDefinition("Dependent", DriverDataType.Int32,
+                """return (int)ctx.GetTag("Target").Value + 1;""", ChangeTriggered: true),
+            new VirtualTagDefinition("Writer", DriverDataType.Int32,
+                """
+                var v = (int)ctx.GetTag("In").Value;
+                ctx.SetVirtualTag("Target", v * 100);
+                return v;
+                """),
+        ]);
+        await engine.EvaluateAllAsync(TestContext.Current.CancellationToken);
+
+        // Writer set Target = 500; the cascade must have re-evaluated Dependent to 501.
+        engine.Read("Target").Value.ShouldBe(500);
+        await WaitForConditionAsync(() => Equals(engine.Read("Dependent").Value, 501));
+        engine.Read("Dependent").Value.ShouldBe(501,
+            "a ctx.SetVirtualTag write must cascade to change-triggered dependents");
+    }
+
     [Fact]
     public async Task Type_coercion_from_script_double_to_config_int32()
     {