garden-and-surfaces: drop the authoring probe and the superseded treelines
probe_magenta was the three-cube test that proved a material authored during
play does reach the GPU through the file path. treeline_far/warm/cool were the
hedge-shader version of the estate beyond the wall; beyond_far/warm/cool replaced
them with a pbr material carrying an emissive floor, because a backdrop that is
lit by the garden's own sun is a black slab at whichever end of the lawn is
turned away from it.
