chunks build two a tick, nearest first, after the first window, so a new row doesn't cost one long frame. a pick's forgiveness shrinks with distance, where small things crowd and a tap mostly means the ground. off-screen critters send nothing. tests: a far tap near the horizon is run to and the camera follows; chunks are the same for a seed and stream in and out deterministically; a run over 30+ chunk builds keeps the program under 1800 numbers; the blanket calls it home; flower, butterfly, stream, tall grass and sheep each do their thing. run-perf.mjs times JS per frame at phone size, resting and running.