This allowed to rewrite implementation in a much clearer way. One user-visible change, however, is that NEW/BASE is now always printed with an index even if there is only one (implementation complexity to accommodate empty index is not worth it).
Ultimately, it turned out that the approach of the old save file format of separating runs metadatas and results only leads to super complicated code. Unfortunately, it didn't help with append-updates to save files (they are still loaded fully, their content is modified in memory, and the actual file is then overwritten).
Timofey Solomko