feat(16-02): implement consolidated HTML rendering path

- Add mergePermissions parameter to BuildHtml and WriteAsync
- Early-return branch calls PermissionConsolidator.Consolidate and delegates to BuildConsolidatedHtml
- BuildConsolidatedHtml: by-user table with Sites column, expandable [N sites] badge with toggleGroup, hidden sub-rows (data-group=locN), inline title for single-location entries
- By-site view and btn-site omitted when mergePermissions=true
- Wire UserAccessAuditViewModel.ExportHtmlAsync to pass MergePermissions
- Fix existing branding test call site to use named parameter

SharepointToolbox.Tests/Services/Export/UserAccessHtmlExportServiceTests.cs

@@ -138,7 +138,7 @@ public class UserAccessHtmlExportServiceTests
     public void BuildHtml_WithBranding_ContainsLogoImg()
     {
         var svc = new UserAccessHtmlExportService();
-        var html = svc.BuildHtml(new[] { DefaultEntry }, MakeBranding(msp: true));
+        var html = svc.BuildHtml(new[] { DefaultEntry }, branding: MakeBranding(msp: true));
         Assert.Contains("data:image/png;base64,bXNw", html);
     }