Single point of serialization for asobi_world_lobby:find_or_create/1.

The naive find_or_create implementation has a TOCTOU race: two clients calling at the same time both see list_worlds(...) = [] because neither has finished create_world/1 yet, and both end up spawning a new world for the same mode. Customer-visible symptom: two players opening barrow at the same instant land in different hub worlds and never see each other.

This gen_server forces all find_or_create calls through a single process. The handler runs the existing list_worlds + create_world sequence atomically with respect to other callers, so the second caller sees the world the first one just spawned.

Calls are sequential, but create_world/1 is the slowest step and takes <100ms in practice; for the small number of distinct modes a typical deployment supports, the queue stays empty.

It also owns the protected ETS table backing the cached discovery listings for both worlds and matches. Callers read the table directly (protected reads); only this server writes, via cache_listing/3.