feat(site-runtime): InstanceActor trace arm stamps the device echo of a traced write; Trace rides on published changes

Flow Debugger (design 2026-09-05 §5.3): a traced SetStaticAttributeCommand on a data-sourced
attribute records (attribute → writer, expiry) for TraceArmWindowSeconds (new option,
default 30, validated > 0); the next DCL echo for that attribute is published with the
writer's SourceExecutionId + Trace whatever its value, and the arm is consumed once. Static
writes publish Trace directly. Untraced writes never arm (pinned).

Claude-Session: https://claude.ai/code/session_01GSLkkHKz9JrAtmJjmQK3cP

src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/InstanceActor.cs

@@ -172,6 +172,15 @@ public class InstanceActor : ReceiveActor
 
     // DCL manager actor reference for subscribing to tag values
     private readonly IActorRef? _dclManager;
+
+    /// <summary>
+    /// Flow Debugger trace arm (design 2026-09-05 §5.3): attribute → (writer's ExecutionId,
+    /// expiry) recorded when a TRACED script write targets a data-sourced attribute. The next
+    /// DCL value change for that attribute is published stamped with the writer's id and
+    /// <c>Trace</c>, and the arm is consumed. Bounded by the attribute count (one entry per
+    /// attribute, newest wins); no sweep — an unconsumed arm is overwritten or expires in place.
+    /// </summary>
+    private readonly Dictionary<string, (Guid SourceExecutionId, DateTimeOffset ExpiresAt)> _traceArms = new();
     // Maps each tag path to every attribute canonical name that references it.
     // A tag path can back more than one attribute (e.g. two composed modules
     // whose members reference the same PLC node), so a tag value update must
@@ -592,7 +601,10 @@ public class InstanceActor : ReceiveActor
             // execution's id onto the change so a child Alarm Actor this write
             // trips can chain its on-trigger script run under the writer. Null for
             // writes with no audited origin (central Test Run, deployment tooling).
-            SourceExecutionId: command.SourceExecutionId);
+            SourceExecutionId: command.SourceExecutionId,
+            // Flow Debugger (design §5.2): a traced write's change carries the flag so the
+            // value-change run it spawns is traced and parented to the writer.
+            Trace: command.Trace);
 
         PublishAndNotifyChildren(changed);
 
@@ -742,6 +754,14 @@ public class InstanceActor : ReceiveActor
             writeValue,
             DateTimeOffset.UtcNow);
 
+        // Flow Debugger trace arm (design §5.3): only a TRACED write with a known writer arms;
+        // the device echo that follows is stamped in HandleTagValueUpdate. Untraced writes
+        // never touch the map, so production behaviour is unchanged.
+        if (command.Trace && command.SourceExecutionId is { } traceSource)
+        {
+            _traceArms[attributeName] = (traceSource, DateTimeOffset.UtcNow.AddSeconds(_options.TraceArmWindowSeconds));
+        }
+
         // Ask the DCL and pipe the result back to the original caller. The DCL
         // returns the failure synchronously so the script can handle it.
         _dclManager.Ask<WriteTagResponse>(writeRequest, TimeSpan.FromSeconds(30))
@@ -1035,6 +1055,17 @@ public class InstanceActor : ReceiveActor
             // O(1) lookup off the hot ingest path (was a linear FirstOrDefault).
             _resolvedAttributeByName.TryGetValue(attrName, out var resolved);
 
+            // Flow Debugger trace arm (design §5.3): consume the arm for this attribute — the
+            // echo is stamped whatever its value (a device that clamps or rejects the write
+            // still shows what it did) — unless the arm has already expired.
+            Guid? traceSource = null;
+            var traced = false;
+            if (_traceArms.Remove(attrName, out var arm) && arm.ExpiresAt > DateTimeOffset.UtcNow)
+            {
+                traceSource = arm.SourceExecutionId;
+                traced = true;
+            }
+
             // A List-typed attribute coerces the incoming OPC UA array
             // (a CLR array/IEnumerable from the SDK) into a typed List<T>. On an
             // element-type mismatch we set the attribute's quality to Bad, log a
@@ -1045,7 +1076,8 @@ public class InstanceActor : ReceiveActor
                 {
                     HandleAttributeValueChanged(new AttributeValueChanged(
                         _instanceUniqueName, update.TagPath, attrName,
-                        typedList, QualityToString(update.Quality), update.Timestamp));
+                        typedList, QualityToString(update.Quality), update.Timestamp,
+                        SourceExecutionId: traceSource, Trace: traced));
                 }
                 else
                 {
@@ -1062,7 +1094,8 @@ public class InstanceActor : ReceiveActor
                     // non-change.
                     PublishAndNotifyChildren(new AttributeValueChanged(
                         _instanceUniqueName, update.TagPath, attrName,
-                        currentValue, "Bad", update.Timestamp), evaluateWaiters: false);
+                        currentValue, "Bad", update.Timestamp,
+                        SourceExecutionId: traceSource, Trace: traced), evaluateWaiters: false);
                 }
                 continue;
             }
@@ -1076,7 +1109,8 @@ public class InstanceActor : ReceiveActor
 
             HandleAttributeValueChanged(new AttributeValueChanged(
                 _instanceUniqueName, update.TagPath, attrName,
-                value, QualityToString(update.Quality), update.Timestamp));
+                value, QualityToString(update.Quality), update.Timestamp,
+                SourceExecutionId: traceSource, Trace: traced));
         }
     }