Fix storage metrics not being accurate

SharepointToolbox/Services/Export/ExportFileWriter.cs

@@ -24,4 +24,41 @@ internal static class ExportFileWriter
     /// <summary>Writes <paramref name="html"/> to <paramref name="filePath"/> as UTF-8 without BOM.</summary>
     public static Task WriteHtmlAsync(string filePath, string html, CancellationToken ct)
         => File.WriteAllTextAsync(filePath, html, Utf8NoBom, ct);
+
+    /// <summary>
+    /// Streams a <see cref="StringBuilder"/> directly to disk as UTF-8 with
+    /// BOM, chunk by chunk. Avoids the full-document <c>ToString()</c> copy
+    /// and the separate UTF-8 byte buffer that <see cref="File.WriteAllTextAsync(string, string, Encoding, CancellationToken)"/>
+    /// would otherwise allocate — meaningful for large CSV exports.
+    /// </summary>
+    public static Task WriteCsvChunksAsync(string filePath, StringBuilder builder, CancellationToken ct)
+        => WriteChunksAsync(filePath, builder, Utf8WithBom, ct);
+
+    /// <summary>
+    /// Streams a <see cref="StringBuilder"/> directly to disk as UTF-8 without
+    /// BOM. Same rationale as <see cref="WriteCsvChunksAsync"/> — for large
+    /// HTML reports it halves peak memory by skipping the intermediate string.
+    /// </summary>
+    public static Task WriteHtmlChunksAsync(string filePath, StringBuilder builder, CancellationToken ct)
+        => WriteChunksAsync(filePath, builder, Utf8NoBom, ct);
+
+    private static async Task WriteChunksAsync(string filePath, StringBuilder builder, Encoding encoding, CancellationToken ct)
+    {
+        // FileOptions.Asynchronous lets StreamWriter use true async I/O.
+        await using var fs = new FileStream(
+            filePath,
+            FileMode.Create,
+            FileAccess.Write,
+            FileShare.None,
+            bufferSize: 64 * 1024,
+            options: FileOptions.Asynchronous | FileOptions.SequentialScan);
+        await using var sw = new StreamWriter(fs, encoding, bufferSize: 64 * 1024);
+
+        foreach (var chunk in builder.GetChunks())
+        {
+            ct.ThrowIfCancellationRequested();
+            await sw.WriteAsync(chunk, ct);
+        }
+        await sw.FlushAsync(ct);
+    }
 }