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/Export/StorageCsvExportServiceTests.cs

@@ -0,0 +1,52 @@
+using SharepointToolbox.Core.Models;
+using SharepointToolbox.Services.Export;
+using Xunit;
+
+namespace SharepointToolbox.Tests.Services.Export;
+
+public class StorageCsvExportServiceTests
+{
+    [Fact]
+    public void BuildCsv_WithKnownNodes_ProducesHeaderRow()
+    {
+        var svc = new StorageCsvExportService();
+        var nodes = new List<StorageNode>
+        {
+            new() { Name = "Shared Documents", Library = "Shared Documents", SiteTitle = "MySite",
+                    TotalSizeBytes = 1024, FileStreamSizeBytes = 800, TotalFileCount = 5,
+                    LastModified = new DateTime(2024, 1, 15) }
+        };
+        var csv = svc.BuildCsv(nodes);
+        Assert.Contains("Library", csv);
+        Assert.Contains("Site", csv);
+        Assert.Contains("Files", csv);
+        Assert.Contains("Total Size", csv);
+        Assert.Contains("Version Size", csv);
+        Assert.Contains("Last Modified", csv);
+    }
+
+    [Fact]
+    public void BuildCsv_WithEmptyList_ReturnsHeaderOnly()
+    {
+        var svc = new StorageCsvExportService();
+        var csv = svc.BuildCsv(new List<StorageNode>());
+        Assert.NotEmpty(csv); // must have at least the header row
+        var lines = csv.Split('\n', StringSplitOptions.RemoveEmptyEntries);
+        Assert.Single(lines); // only header, no data rows
+    }
+
+    [Fact]
+    public void BuildCsv_NodeValues_AppearInOutput()
+    {
+        var svc = new StorageCsvExportService();
+        var nodes = new List<StorageNode>
+        {
+            new() { Name = "Reports", Library = "Reports", SiteTitle = "ProjectSite",
+                    TotalSizeBytes = 2048, FileStreamSizeBytes = 1024, TotalFileCount = 10 }
+        };
+        var csv = svc.BuildCsv(nodes);
+        Assert.Contains("Reports", csv);
+        Assert.Contains("ProjectSite", csv);
+        Assert.Contains("10", csv);
+    }
+}