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.