test(02-01): scaffold PermissionsService, ViewModel, and classification test stubs

- PermissionEntryHelper.cs: pure static IsExternalUser, FilterPermissionLevels, IsSharingLinksGroup
- PermissionEntryClassificationTests.cs: 7 real [Fact] tests — all passing immediately
- PermissionsServiceTests.cs: 2 stubs (PERM-01, PERM-04) skipped until Plan 02 CSOM impl
- PermissionsViewModelTests.cs: 1 stub (PERM-02) skipped until Plan 02 ViewModel impl

SharepointToolbox.Tests/ViewModels/PermissionsViewModelTests.cs

@@ -0,0 +1,22 @@
+using SharepointToolbox.Core.Models;
+using SharepointToolbox.Services;
+
+namespace SharepointToolbox.Tests.ViewModels;
+
+/// <summary>
+/// Test stubs for PERM-02 (multi-site scan loop).
+/// Skipped until PermissionsViewModel is implemented in Plan 02.
+/// </summary>
+public class PermissionsViewModelTests
+{
+    [Fact(Skip = "Requires live CSOM context — covered by Plan 02 implementation")]
+    public async Task StartScanAsync_WithMultipleSiteUrls_CallsServiceOncePerUrl()
+    {
+        // PERM-02: When the user supplies N site URLs, IPermissionsService.ScanSiteAsync
+        // is invoked exactly once per URL (sequential, not parallel).
+        // Arrange — requires PermissionsViewModel and a mock IPermissionsService
+        // Act
+        // Assert
+        await Task.CompletedTask;
+    }
+}