A pipeline stage: a Shader, or WebGPU-style { module = Shader, entryPoint = name }. The Shader’s @vertex/@fragment functions are the
selectable entry points; omit entryPoint to use the first one of that
stage (like WebGPU).
Vertex and fragment may come from different Shaders. Each file’s bindings
are numbered on its own, so on Vulkan, D3D12 and GL — which share one slot
namespace between the stages — two files that declare different bindings
can collide; GPUPipeline.new rejects that case naming both bindings.
Declaring the same bindings in both files always works.
