The device backends. The proprietary, hardware-facing renderers - Metal
(macOS), DirectX 12 (Windows), Vulkan (Windows/Linux) - plus the shared native
Win32 window/input layer. Exactly one backend compiles per build (resolved by
build.rs into a single backend_* cfg). Depends on concinnity-render (the
RenderBackend/SceneControl trait seam + render-prep) and concinnity-core;
owns no gameplay, ECS-runtime, audio, or physics. The client drives these through a
Box<dyn RenderBackend> obtained from init_backend, never naming a concrete
context type.