feat(09-01): extend IStorageService with CollectFileTypeMetricsAsync

- Add CollectFileTypeMetricsAsync method signature to IStorageService
- Returns IReadOnlyList<FileTypeMetric> for chart visualization data
- Existing CollectStorageAsync signature unchanged
- CS0535 expected until StorageService implements in Plan 09-02

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

SharepointToolbox/Services/IStorageService.cs

@@ -5,9 +5,25 @@ namespace SharepointToolbox.Services;
 
 public interface IStorageService
 {
+    /// <summary>
+    /// Collects storage metrics per library/folder using SharePoint StorageMetrics API.
+    /// Returns a tree of StorageNode objects with aggregate size data.
+    /// </summary>
     Task<IReadOnlyList<StorageNode>> CollectStorageAsync(
         ClientContext ctx,
         StorageScanOptions options,
         IProgress<OperationProgress> progress,
         CancellationToken ct);
+
+    /// <summary>
+    /// Enumerates files across all non-hidden document libraries in the site
+    /// and aggregates storage consumption grouped by file extension.
+    /// Uses CSOM CamlQuery to retrieve FileLeafRef and File_x0020_Size per file.
+    /// This is a separate operation from CollectStorageAsync -- it provides
+    /// file-type breakdown data for chart visualization.
+    /// </summary>
+    Task<IReadOnlyList<FileTypeMetric>> CollectFileTypeMetricsAsync(
+        ClientContext ctx,
+        IProgress<OperationProgress> progress,
+        CancellationToken ct);
 }