Registry of game-defined protobuf schemas for entity metadata.

Metadata is stored and served as JSON (database, REST API, admin UI), but realtime pushes on protobuf-format connections can carry it as compact binary when the game registers a schema. Registration is convention-based: when a plugin loads, its modules are scanned for protobuf messages named UserMeta, LobbyMeta, GroupMeta or PartyMeta (any namespace), which are registered for the matching entity automatically. A plugin can override or disable the convention by exporting metadata_schemas/0 returning e.g. %{user: MyGame.Proto.Profile, lobby: nil}.

Entity metadata schemas are global (one per deployment): unlike hook schemas, which are namespaced per plugin, every plugin contributes to the same four entity slots. On conflict the precedence is well-defined: explicit metadata_schemas/0 entries beat name conventions, and within the same priority the first plugin in name order wins; every losing registration is logged. An explicit nil disables the entity globally (sticky — conventions from other plugins cannot re-add it).

Lookups run on every push, so the registry lives in :persistent_term (refreshed by Gamend.Hooks.PluginManager on every plugin reload).