feat(03-01): create Phase 3 export stubs and test scaffolds

- Add StorageCsvExportService, StorageHtmlExportService stub (Plan 03-03)
- Add SearchCsvExportService, SearchHtmlExportService stub (Plan 03-05)
- Add DuplicatesHtmlExportService stub (Plan 03-05)
- Add StorageServiceTests, SearchServiceTests, DuplicatesServiceTests scaffolds
- Add export test scaffolds for all 4 Phase 3 export services
- 7 pure-logic tests pass (VersionSizeBytes + MakeKey); 4 CSOM stubs skip

SharepointToolbox.Tests/Services/SearchServiceTests.cs

@@ -0,0 +1,20 @@
+using SharepointToolbox.Core.Models;
+using SharepointToolbox.Services;
+using Xunit;
+
+namespace SharepointToolbox.Tests.Services;
+
+public class SearchServiceTests
+{
+    [Fact(Skip = "Requires live CSOM context — covered by Plan 03-04 implementation")]
+    public Task SearchFilesAsync_WithExtensionFilter_BuildsCorrectKql()
+        => Task.CompletedTask;
+
+    [Fact(Skip = "Requires live CSOM context — covered by Plan 03-04 implementation")]
+    public Task SearchFilesAsync_PaginationStopsAt50000()
+        => Task.CompletedTask;
+
+    [Fact(Skip = "Requires live CSOM context — covered by Plan 03-04 implementation")]
+    public Task SearchFilesAsync_FiltersVersionHistoryPaths()
+        => Task.CompletedTask;
+}