Game subsystem: persistent game manager with auto-init

SETUP: Create Blueprint inheriting UGameInstanceSubsystem (or UWorldSubsystem for level-scoped). Override Initialize(Collection) to set up. Override Deinitialize to clean up. Add game variables here (Score, PlayTime, Settings).

ACCESS FROM ANYWHERE: GetGameInstance → GetSubsystem(BP_GameSubsystem) → CastTo<BP_GameSubsystem> → access variables.

WHY: Subsystems auto-create, auto-destroy, and don't need FindActor. Better than GameInstance for well-organized systems. UGameInstanceSubsystem persists across levels. UWorldSubsystem is recreated each level.