Function editor_wgpu_backends 

pub fn editor_wgpu_backends() -> Backends

wgpu backends for an editor surface embedded in a host-owned child window. Mirror of truce_gui::platform::editor_wgpu_backends (truce-gpu can’t depend on truce-gui without a dep cycle). Windows is DX12 (the only backend feature compiled in on Windows), macOS is Metal, Linux keeps PRIMARY. Keep the two copies in sync.