feat(DbExporter): add counting data reader for accurate row count

Wrap IDataReader with CountingDataReader to track rows as they're read
during protobuf serialization, fixing the export returning 0 rows.

Tools/DbExporter/DatabaseExporter.cs

@@ -26,7 +26,8 @@ public sealed class DatabaseExporter
         command.CommandText = definition.Query;
         command.CommandTimeout = 0; // No timeout for large exports
 
-        await using var reader = await command.ExecuteReaderAsync(cancellationToken);
+        await using var baseReader = await command.ExecuteReaderAsync(cancellationToken);
+        var reader = new CountingDataReader(baseReader);
 
         long uncompressedSize = 0;
 
@@ -51,9 +52,7 @@ public sealed class DatabaseExporter
 
         var compressedSize = new FileInfo(definition.OutputPath).Length;
 
-        // Row count requires a separate pass or we estimate from verify
-        // Return 0 for now, verify will get accurate count
-        return new ExportResult(0, uncompressedSize, compressedSize, hash);
+        return new ExportResult(reader.RowCount, uncompressedSize, compressedSize, hash);
     }
 
     private static DbConnection CreateConnection(string providerType, string connectionString)