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.

src/ScadaLink.Commons/Interfaces/Services/INodeIdentityProvider.cs

@@ -0,0 +1,22 @@
+namespace ScadaLink.Commons.Interfaces.Services;
+
+/// <summary>
+/// Surfaces the local node's semantic role-within-cluster name so downstream
+/// audit writers can stamp it on the SourceNode column.
+/// </summary>
+/// <remarks>
+/// Conventional values follow the pattern <c>node-a</c>/<c>node-b</c> on site
+/// nodes and <c>central-a</c>/<c>central-b</c> on central nodes. The value is
+/// a free-form operator-supplied label — there is no enforced format. When the
+/// configuration value is missing, empty, or whitespace, implementations
+/// return <c>null</c> so audit writers can persist NULL rather than an empty
+/// string.
+/// </remarks>
+public interface INodeIdentityProvider
+{
+    /// <summary>
+    /// The configured semantic node name, trimmed of surrounding whitespace.
+    /// <c>null</c> when unconfigured.
+    /// </summary>
+    string? NodeName { get; }
+}