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.SiteRuntime.Tests/Scripts/ExternalSystemCachedCallEmissionTests.cs

@@ -570,4 +570,69 @@ public class ExternalSystemCachedCallEmissionTests
         var only = Assert.Single(forwarder.Telemetry);
         Assert.Equal(AuditKind.CachedSubmit, only.Audit.Kind);
     }
+
+    // ── SourceNode-stamping (Task 14) ──
+
+    [Fact]
+    public async Task CachedCall_StampsSourceNode_OnEverySiteCallOperationalRow()
+    {
+        // SourceNode-stamping (Task 14): when the helper is constructed with
+        // a non-null sourceNode, every SiteCallOperational it produces
+        // (CachedSubmit on enqueue + the immediate-completion Attempted/
+        // CachedResolve pair when WasBuffered=false) carries that node name.
+        var client = new Mock<IExternalSystemClient>();
+        client
+            .Setup(c => c.CachedCallAsync(
+                "ERP", "GetOrder",
+                It.IsAny<IReadOnlyDictionary<string, object?>?>(),
+                InstanceName,
+                It.IsAny<CancellationToken>(),
+                It.IsAny<TrackedOperationId?>(),
+                It.IsAny<Guid?>(), It.IsAny<string?>(), It.IsAny<Guid?>()))
+            // Immediate completion — helper produces all three rows itself.
+            .ReturnsAsync(new ExternalCallResult(true, "{\"ok\":true}", null, WasBuffered: false));
+        var forwarder = new CapturingForwarder();
+
+        var helper = new ScriptRuntimeContext.ExternalSystemHelper(
+            client.Object,
+            InstanceName,
+            NullLogger.Instance,
+            TestExecutionId,
+            auditWriter: null,
+            siteId: SiteId,
+            sourceScript: SourceScript,
+            cachedForwarder: forwarder,
+            parentExecutionId: null,
+            sourceNode: "node-a");
+
+        await helper.CachedCall("ERP", "GetOrder");
+
+        Assert.Equal(3, forwarder.Telemetry.Count);
+        Assert.All(forwarder.Telemetry, t => Assert.Equal("node-a", t.Operational.SourceNode));
+    }
+
+    [Fact]
+    public async Task CachedCall_NoSourceNodeWired_LeavesSourceNodeNull()
+    {
+        // Default CreateHelper does NOT pass sourceNode — the legacy / test
+        // host path. Every operational row carries null SourceNode, leaving
+        // central's SiteCalls.SourceNode NULL.
+        var client = new Mock<IExternalSystemClient>();
+        client
+            .Setup(c => c.CachedCallAsync(
+                "ERP", "GetOrder",
+                It.IsAny<IReadOnlyDictionary<string, object?>?>(),
+                InstanceName,
+                It.IsAny<CancellationToken>(),
+                It.IsAny<TrackedOperationId?>(),
+                It.IsAny<Guid?>(), It.IsAny<string?>(), It.IsAny<Guid?>()))
+            .ReturnsAsync(new ExternalCallResult(true, null, null, WasBuffered: true));
+        var forwarder = new CapturingForwarder();
+
+        var helper = CreateHelper(client.Object, forwarder);
+        await helper.CachedCall("ERP", "GetOrder");
+
+        var only = Assert.Single(forwarder.Telemetry);
+        Assert.Null(only.Operational.SourceNode);
+    }
 }