test(sitecall-audit): symmetric SourceNode coverage on DbOutbound emitter + clarify DI comments

Two follow-ups from the T13/T14 code review:

- M1: Add CachedWrite_StampsSourceNode_OnSubmitTelemetryRow and
  CachedWrite_NoSourceNodeWired_LeavesSourceNodeNull to DatabaseCachedWriteEmissionTests,
  mirroring the existing ApiOutbound SourceNode tests in
  ExternalSystemCachedCallEmissionTests. Site-emitter coverage now symmetric
  across both cached-call channels.
- M2: Clarify the GetService(INodeIdentityProvider) DI comments on the
  CachedCallTelemetryForwarder and CachedCallLifecycleBridge factories:
  it's test composition roots that may not register the provider, not
  central production. Both site and central hosts always register it via
  SiteServiceRegistration.BindSharedOptions.

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

@@ -328,4 +328,68 @@ public class DatabaseCachedWriteEmissionTests
             It.IsAny<Guid?>(), It.IsAny<string?>(), It.IsAny<Guid?>()),
             Times.Once);
     }
+
+    // ── SourceNode-stamping (Task 14) ──
+
+    [Fact]
+    public async Task CachedWrite_StampsSourceNode_OnSubmitTelemetryRow()
+    {
+        // Symmetric to ExternalSystemCachedCallEmissionTests's
+        // CachedCall_StampsSourceNode_OnEverySiteCallOperationalRow — locks
+        // the DbOutbound emitter against a future refactor that drops
+        // _sourceNode from the Database.CachedWrite CachedSubmit row.
+        var gateway = new Mock<IDatabaseGateway>();
+        gateway
+            .Setup(g => g.CachedWriteAsync(
+                "myDb", "INSERT INTO t VALUES (1)",
+                It.IsAny<IReadOnlyDictionary<string, object?>?>(),
+                InstanceName,
+                It.IsAny<CancellationToken>(),
+                It.IsAny<TrackedOperationId?>(),
+                It.IsAny<Guid?>(), It.IsAny<string?>(), It.IsAny<Guid?>()))
+            .Returns(Task.CompletedTask);
+        var forwarder = new CapturingForwarder();
+
+        var helper = new ScriptRuntimeContext.DatabaseHelper(
+            gateway.Object,
+            InstanceName,
+            NullLogger.Instance,
+            TestExecutionId,
+            auditWriter: null,
+            siteId: SiteId,
+            sourceScript: SourceScript,
+            cachedForwarder: forwarder,
+            parentExecutionId: null,
+            sourceNode: "node-a");
+
+        await helper.CachedWrite("myDb", "INSERT INTO t VALUES (1)");
+
+        var packet = Assert.Single(forwarder.Telemetry);
+        Assert.Equal("node-a", packet.Operational.SourceNode);
+    }
+
+    [Fact]
+    public async Task CachedWrite_NoSourceNodeWired_LeavesSourceNodeNull()
+    {
+        // Default CreateHelper does NOT pass sourceNode — the legacy / test
+        // host path. The operational row carries null SourceNode, leaving
+        // central's SiteCalls.SourceNode NULL.
+        var gateway = new Mock<IDatabaseGateway>();
+        gateway
+            .Setup(g => g.CachedWriteAsync(
+                "myDb", "INSERT INTO t VALUES (1)",
+                It.IsAny<IReadOnlyDictionary<string, object?>?>(),
+                InstanceName,
+                It.IsAny<CancellationToken>(),
+                It.IsAny<TrackedOperationId?>(),
+                It.IsAny<Guid?>(), It.IsAny<string?>(), It.IsAny<Guid?>()))
+            .Returns(Task.CompletedTask);
+        var forwarder = new CapturingForwarder();
+
+        var helper = CreateHelper(gateway.Object, forwarder);
+        await helper.CachedWrite("myDb", "INSERT INTO t VALUES (1)");
+
+        var packet = Assert.Single(forwarder.Telemetry);
+        Assert.Null(packet.Operational.SourceNode);
+    }
 }