Top-level world supervisor.

Public ETS trust assumption (F-33): asobi_world_state and asobi_player_worlds are public named ETS tables. Anything running in the same BEAM (game callbacks, plugins, etc.) can read and mutate them. asobi is single-tenant and the loaded code is trusted, so this is acceptable — but it's an explicit trust boundary worth surfacing in the threat model. Any sandboxed runtime layered on top of asobi (e.g. asobi_lua) MUST keep its sandbox out of these tables.