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/StorageServiceTests.cs

@@ -0,0 +1,31 @@
+using SharepointToolbox.Core.Models;
+using SharepointToolbox.Services;
+using Xunit;
+
+namespace SharepointToolbox.Tests.Services;
+
+public class StorageServiceTests
+{
+    [Fact(Skip = "Requires live CSOM context — covered by Plan 03-02 implementation")]
+    public Task CollectStorageAsync_ReturnsLibraryNodes_ForDocumentLibraries()
+        => Task.CompletedTask;
+
+    [Fact(Skip = "Requires live CSOM context — covered by Plan 03-02 implementation")]
+    public Task CollectStorageAsync_WithFolderDepth1_ReturnsSubfolderNodes()
+        => Task.CompletedTask;
+
+    [Fact]
+    public void StorageNode_VersionSizeBytes_IsNonNegative()
+    {
+        // VersionSizeBytes = TotalSizeBytes - FileStreamSizeBytes (never negative)
+        var node = new StorageNode { TotalSizeBytes = 1000L, FileStreamSizeBytes = 1200L };
+        Assert.Equal(0L, node.VersionSizeBytes); // Math.Max(0, -200) = 0
+    }
+
+    [Fact]
+    public void StorageNode_VersionSizeBytes_IsCorrectWhenPositive()
+    {
+        var node = new StorageNode { TotalSizeBytes = 5000L, FileStreamSizeBytes = 3000L };
+        Assert.Equal(2000L, node.VersionSizeBytes);
+    }
+}