feat(sitecall-audit): carry + persist SourceNode end-to-end via cached telemetry

Site: site emitters of SiteCallOperational (ExternalSystemClient, the script-API
cached call path in ScriptRuntimeContext, CachedCallLifecycleBridge) inject
INodeIdentityProvider and stamp SourceNode = NodeName at construction.

OperationTrackingStore call site in CachedCallTelemetryForwarder now stamps
SourceNode too.

Central: SiteCallAuditRepository.UpsertAsync INSERT includes SourceNode in the
column list; conditional monotonic UPDATE uses
COALESCE(@SourceNode, SourceNode) so later packets cannot blank a previously-
stamped value. After this commit every SiteCalls row carries node-a/node-b in
SourceNode (subject to monotonic preservation).

tests/ScadaLink.AuditLog.Tests/Site/Telemetry/CachedCallTelemetryForwarderTests.cs

@@ -135,7 +135,11 @@ public class CachedCallTelemetryForwarderTests
             Arg.Any<CancellationToken>());
 
         // Tracking row: insert-if-not-exists with kind discriminator.
-        // sourceNode is null until Task 14 wires the INodeIdentityProvider through.
+        // Default CreateSut() does NOT supply an INodeIdentityProvider, so the
+        // forwarder passes null sourceNode to RecordEnqueueAsync (legacy / test
+        // host behaviour). The Task 14 stamping path is covered by the
+        // ForwardAsync_Submit_StampsSourceNode_FromNodeIdentityProvider test
+        // below.
         await _tracking.Received(1).RecordEnqueueAsync(
             _id,
             "ApiOutbound",
@@ -249,4 +253,55 @@ public class CachedCallTelemetryForwarderTests
         await Assert.ThrowsAsync<ArgumentNullException>(
             () => sut.ForwardAsync(null!, CancellationToken.None));
     }
+
+    // ── SourceNode-stamping (Task 14) ──
+
+    [Fact]
+    public async Task ForwardAsync_Submit_StampsSourceNode_FromNodeIdentityProvider()
+    {
+        // SourceNode-stamping (Task 14): when an INodeIdentityProvider is
+        // wired the forwarder must stamp its NodeName onto the
+        // RecordEnqueueAsync sourceNode parameter so the tracking row
+        // captures the originating node (node-a/node-b).
+        var nodeIdentity = Substitute.For<INodeIdentityProvider>();
+        nodeIdentity.NodeName.Returns("node-a");
+
+        var sut = new CachedCallTelemetryForwarder(
+            _writer, _tracking, NullLogger<CachedCallTelemetryForwarder>.Instance, nodeIdentity);
+
+        await sut.ForwardAsync(SubmitPacket(), CancellationToken.None);
+
+        await _tracking.Received(1).RecordEnqueueAsync(
+            _id,
+            "ApiOutbound",
+            "ERP.GetOrder",
+            "inst-1",
+            "ScriptActor:doStuff",
+            "node-a",
+            Arg.Any<CancellationToken>());
+    }
+
+    [Fact]
+    public async Task ForwardAsync_Submit_NodeIdentityNullNodeName_PassesNullSourceNode()
+    {
+        // The provider exists but reports a null NodeName (unconfigured).
+        // The forwarder passes that null through to RecordEnqueueAsync rather
+        // than falling back to a placeholder string.
+        var nodeIdentity = Substitute.For<INodeIdentityProvider>();
+        nodeIdentity.NodeName.Returns((string?)null);
+
+        var sut = new CachedCallTelemetryForwarder(
+            _writer, _tracking, NullLogger<CachedCallTelemetryForwarder>.Instance, nodeIdentity);
+
+        await sut.ForwardAsync(SubmitPacket(), CancellationToken.None);
+
+        await _tracking.Received(1).RecordEnqueueAsync(
+            _id,
+            "ApiOutbound",
+            "ERP.GetOrder",
+            "inst-1",
+            "ScriptActor:doStuff",
+            null,
+            Arg.Any<CancellationToken>());
+    }
 }