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/Services/Export/StorageCsvExportService.cs

@@ -0,0 +1,14 @@
+using SharepointToolbox.Core.Models;
+
+namespace SharepointToolbox.Services.Export;
+
+public class StorageCsvExportService
+{
+    public string BuildCsv(IReadOnlyList<StorageNode> nodes) => string.Empty; // implemented in Plan 03-03
+
+    public async Task WriteAsync(IReadOnlyList<StorageNode> nodes, string filePath, CancellationToken ct)
+    {
+        var csv = BuildCsv(nodes);
+        await System.IO.File.WriteAllTextAsync(filePath, csv, new System.Text.UTF8Encoding(true), ct);
+    }
+}