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• Godot uses "startup" and "shutdown", I think we should keep things consistent with the other godot-rust APIs and use "init" and "deinit" instead. Then we'd have on_level_init + on_main_loop_init, etc.
• Is there a strict order between the levels and the main loop? If yes, should we (not in this PR, but a follow-up) consider representing MainLoop as its own InitLevel?

on_main_loop_init feels off, the startup is actually after the MainLoop/SceneTree is created. It is the final step of Main::start() and after all init levels (shutdown is the vice versa, before all init levels).

I think it's better to make them separate functions, keeping on_init_level lean so you're less likely to make mistakes in it.

on_main_loop_init feels off, the startup is actually after the MainLoop/SceneTree is created. It is the final step of Main::start() and after all init levels (shutdown is the vice versa, before all init levels).

So there is a clear order: all levels -> main loop (init).

In my opinion, it's much easier to understand this by having a separate level, as there is only one API rather than "normal init levels + out-of-band main loop that just happens to be another level". We should see this from user perspective, not from the API that GDExtension itself provides us with. But I can address this later.

Thanks a lot!