feat(audit): stamp SourceNode at site SqliteAuditWriter from INodeIdentityProvider

Caller-provided SourceNode wins (preserves reconciled rows from other nodes);
otherwise the writer fills it from the local INodeIdentityProvider.NodeName.
Reads from the provider on every write — singleton lifetime means zero overhead.

tests/ScadaLink.AuditLog.Tests/TestSupport/FakeNodeIdentityProvider.cs

@@ -0,0 +1,20 @@
+using ScadaLink.Commons.Interfaces.Services;
+
+namespace ScadaLink.AuditLog.Tests.TestSupport;
+
+/// <summary>
+/// Test fake for <see cref="INodeIdentityProvider"/>. Returns the configured
+/// <see cref="NodeName"/> verbatim — including <c>null</c> — so tests can
+/// exercise both the "stamped" and "unconfigured" branches of the SourceNode
+/// stamping logic in <see cref="ScadaLink.AuditLog.Site.SqliteAuditWriter"/>
+/// and <see cref="ScadaLink.AuditLog.Central.CentralAuditWriter"/>.
+/// </summary>
+internal sealed class FakeNodeIdentityProvider : INodeIdentityProvider
+{
+    public string? NodeName { get; }
+
+    public FakeNodeIdentityProvider(string? nodeName = null)
+    {
+        NodeName = nodeName;
+    }
+}