ludic/changes/render3d-vulkan-only.md

bump: major type: breaking render3d draws with Vulkan only; its OpenGL backend is gone. gpu.ludic keeps its seam - every gpu_* and u_* call a renderer or a game makes is unchanged - but each now speaks to the Vulkan backend alone: no gl_* entry point is called from the package, and the GLSL source is no longer assembled at run time (the same files are still what ludic-dev shaders compiles to SPIR-V). gpu_request accepts any name and runs Vulkan; there is no fallback, so a machine whose Vulkan cannot start is told why (gpu_fallback_reason, printed) and r3d_init fails. gpu_is_gl() stays, always false, for callers that still ask. The R3D_GLCHECK error checks went with the backend (their messages were built on every call, checked or not). A program that uses Vk.* now also links the runtime's window layer, which still lives with its OpenGL (gl_width, gl_set_drawable, ...); separating the two is the runtime's own decision.