Phase 6 - First pixel (contents being built ahead of it)

orbistoun-gpu + orbistoun-gpu-vulkan + orbistoun-video, and the GUI's output surface. A Vulkan device, swapchain, and enough command translation to service one flip.

This is also where D032's deferred cost comes due: output is produced in the worker while the window lives in the shim, so it needs either a reparented child-owned window or shared images via external-memory extensions. Deferred deliberately - until now the worker produces no video at all.

Observable result: a window with something in it. Also the arrival of framebuffer diffing, the only cheap mechanical correctness oracle this project will ever have (see TESTING.md).