A fill that went wrong took a seven-minute merge to look at again. Now
DR_FILL_DUMP=dir makes the merge write what the filler was given, and
the fill example runs fill_border on that, or a crop of it, on the engine
and writes coarse, each band and the feathered result as PPMs — seconds
per attempt on TensorRT. Both examples take DARKROOM_ORT_DIR as the app
does, and --wait-engines lets a compiling rung finish before timing.
