feat(notification-outbox): populate SourceScript on outbound notifications

FU3: thread the executing script identifier from the script-execution
context down to the Notify outbox API so NotifyTarget.Send stamps
NotificationSubmit.SourceScript instead of leaving it null.

- ScriptRuntimeContext / NotifyHelper / NotifyTarget take an optional
  sourceScript value, carried through to NotificationSubmit.SourceScript.
- ScriptExecutionActor supplies "ScriptActor:<scriptName>", matching the
  Site Event Logging "Source" convention used for script error events.
- AlarmExecutionActor builds the context without the S&F engine, so its
  Notify API is inert; sourceScript defaults to null there.

tests/ScadaLink.SiteRuntime.Tests/Scripts/NotifyHelperTests.cs

@@ -58,13 +58,16 @@ public class NotifyHelperTests : TestKit, IAsyncLifetime, IDisposable
         base.Dispose(disposing);
     }
 
-    private ScriptRuntimeContext.NotifyHelper CreateHelper(IActorRef siteCommunicationActor)
+    private ScriptRuntimeContext.NotifyHelper CreateHelper(
+        IActorRef siteCommunicationActor,
+        string? sourceScript = null)
     {
         return new ScriptRuntimeContext.NotifyHelper(
             _saf,
             siteCommunicationActor,
             "site-7",
             "Plant.Pump3",
+            sourceScript,
             TimeSpan.FromSeconds(3),
             NullLogger.Instance);
     }
@@ -113,6 +116,36 @@ public class NotifyHelperTests : TestKit, IAsyncLifetime, IDisposable
         Assert.Equal("Plant.Pump3", payload.SourceInstanceId);
     }
 
+    [Fact]
+    public async Task Send_WhenHelperHasSourceScript_StampsItOnTheNotificationSubmit()
+    {
+        var commProbe = CreateTestProbe();
+        var notify = CreateHelper(commProbe.Ref, sourceScript: "ScriptActor:MonitorSpeed");
+
+        var notificationId = await notify.To("Operators").Send("Pump alarm", "Pump 3 tripped");
+
+        var buffered = await _saf.GetMessageByIdAsync(notificationId);
+        Assert.NotNull(buffered);
+
+        var payload = JsonSerializer.Deserialize<NotificationSubmit>(buffered!.PayloadJson);
+        Assert.NotNull(payload);
+        // FU3: the executing script name is threaded down and stamped for the audit trail.
+        Assert.Equal("ScriptActor:MonitorSpeed", payload!.SourceScript);
+    }
+
+    [Fact]
+    public async Task Send_WhenHelperHasNoSourceScript_LeavesSourceScriptNull()
+    {
+        var commProbe = CreateTestProbe();
+        var notify = CreateHelper(commProbe.Ref, sourceScript: null);
+
+        var notificationId = await notify.To("Operators").Send("Pump alarm", "Pump 3 tripped");
+
+        var buffered = await _saf.GetMessageByIdAsync(notificationId);
+        var payload = JsonSerializer.Deserialize<NotificationSubmit>(buffered!.PayloadJson);
+        Assert.Null(payload!.SourceScript);
+    }
+
     [Fact]
     public async Task Status_WhenStillBufferedAtSite_ReportsForwarding()
     {