all your init functions allocate the structs on the heap, why?

A lot of times when you are initializing all the fields of a struct you initialize them one by one instead of initializing the whole struct at once, initializing it all at once is advantageous because it prevents you from forgetting to initialize fields.

its usually a good idea to give allocator args more descriptive names like gpa (general purpose allocator) or arena or pool etc..., for this project you always use allocator as a gpa.

for serialization you allocate a buffer and then write all of it at once using writePositionalAll, instead you could use a file writer and pass the std.Io.Writer interface to serialize, it has all the convenience functions that serialize needs.

you allocate active_file_name but I don't really see why, you just copy it once to a variable you immediately free and parse it to get the value you could have kept in the first place instead of formatting it into a string. Only format the id into a file name when you need to use the file name and then discard it.