Write each effect body-only against a preamble the host supplies,
and let the build systems generate what each renderer needs -- a
Swift catalog for OpenGL, a Metal fragment function through glslang
and spirv-cross. Effects react to arriving events.
The staging directory was declared as a plugin output, so SwiftPM
copied it into LumaCore's resource bundle (~3 MB) and shipped it. It is
only scratch space for embedding the agent, so drop it from the outputs.