feat: bundle documents in index optionally

dotnet/MaigoLabs.NeedLe.Common/Types/CompressedInvertedIndex.cs

@@ -6,7 +6,7 @@ namespace MaigoLabs.NeedLe.Common.Types;
 
 public class CompressedInvertedIndex
 {
-    public required string[] documents { get; set; }
+    public required string[]? documents { get; set; }
     public required int[] tokenTypes { get; set; } // Use int values here instead of TokenType enum to avoid JSON serialization issues.
     public required List<int[]>[] tokenReferences { get; set; } // tokenId -> [documentId, start1, end1, start2, end2, ...]
     public required CompressedInvertedIndexTries tries { get; set; }