feat(host): add NodeName to NodeOptions + INodeIdentityProvider

- NodeName: semantic role-within-cluster identifier (node-a/node-b on sites,
  central-a/central-b on central). Bound from ScadaLink:Node:NodeName.
- INodeIdentityProvider exposes the trimmed name (null if unconfigured) so
  downstream audit writers can stamp the new SourceNode column.

tests/ScadaLink.Host.Tests/NodeIdentityProviderTests.cs

@@ -0,0 +1,50 @@
+using Microsoft.Extensions.Options;
+using ScadaLink.Commons.Interfaces.Services;
+
+namespace ScadaLink.Host.Tests;
+
+/// <summary>
+/// Tests for NodeIdentityProvider — surfaces the operator-configured semantic
+/// node name (e.g. node-a / node-b / central-a / central-b) used by downstream
+/// audit writers to stamp the SourceNode column.
+/// </summary>
+public class NodeIdentityProviderTests
+{
+    private static INodeIdentityProvider BuildProvider(string nodeName)
+    {
+        var options = Options.Create(new NodeOptions { NodeName = nodeName });
+        return new NodeIdentityProvider(options);
+    }
+
+    [Fact]
+    public void NodeIdentityProvider_returns_configured_NodeName()
+    {
+        var provider = BuildProvider("central-a");
+
+        Assert.Equal("central-a", provider.NodeName);
+    }
+
+    [Fact]
+    public void NodeIdentityProvider_returns_null_when_NodeName_unset()
+    {
+        var provider = BuildProvider(string.Empty);
+
+        Assert.Null(provider.NodeName);
+    }
+
+    [Fact]
+    public void NodeIdentityProvider_returns_null_when_NodeName_whitespace()
+    {
+        var provider = BuildProvider("   ");
+
+        Assert.Null(provider.NodeName);
+    }
+
+    [Fact]
+    public void NodeIdentityProvider_trims_whitespace()
+    {
+        var provider = BuildProvider("  node-a  ");
+
+        Assert.Equal("node-a", provider.NodeName);
+    }
+}