feat(deploymentmanager): IFlatteningPipeline.PreviewAsync — flatten a template+site without an instance row (create-page bindings)

Claude-Session: https://claude.ai/code/session_01G2GHPnbbKGvyyfbr2XBRVH

src/ZB.MOM.WW.ScadaBridge.DeploymentManager/IFlatteningPipeline.cs

@@ -39,6 +39,21 @@ public interface IFlatteningPipeline
         bool validateScripts = true,
         FlattenSession? session = null);
 
+    /// <summary>
+    /// Flattens <paramref name="templateId"/> as if an instance with no overrides and no
+    /// bindings existed at <paramref name="siteId"/>. Attribute list only — no validation,
+    /// no revision hash, nothing persisted. Used by the Create Instance page to show the
+    /// module groups before the instance row exists.
+    /// </summary>
+    /// <param name="templateId">Id of the template to preview.</param>
+    /// <param name="siteId">Id of the target site, whose data connections resolve references.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>The flattened configuration of the hypothetical instance, or a failure result.</returns>
+    Task<Result<FlattenedConfiguration>> PreviewAsync(
+        int templateId,
+        int siteId,
+        CancellationToken cancellationToken = default);
+
     /// <summary>
     /// Creates a fresh <see cref="FlattenSession"/> bound to this pipeline's
     /// watermark, for a caller that wants to share one across a batch of