The atomic .tmp→rename pattern caused silent FileNotFoundError on some
bodies. Removed in favour of direct writes — resume logic already handles
interrupted runs. Fixed --heightmap-size CLI flag which was silently
ignored due to Python default parameter binding. Changed default heightmap
resolution from 4096x2048 to 1024x512 (native simulation grid — no
information gain from upscaling).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>