Backend policy

RhiBackendPolicy selects one live backend for a composed viewport set:

Workbench -> QRhi -> D3D11 | Vulkan | Metal

The runtime backend is independent of the generated-code viewer. Windows D3D11 and Vulkan have draw-recording smoke coverage. Linux/macOS wheel import validates package construction, not equivalent runtime rendering.

Implementation

Relevant source: rendering/RhiBackendPolicy.* and app/main.cpp.