Internal
Created by ShaderGraphBuilder only.
The fragment result — vec4, required before the graph can build.
Object-space vec3 displacement added to position — surface graphs
only, optional. Not a transform: §42's authority model is untouched and
the physics world is not told (RFC 0001 Q4, decided).
The two output slots of a graph under construction — assigned, RFC 0001's spelling, as properties:
builder.output.color = expr.