draw_sprites.py renderers take scale/smooth/src_dir/out_dir keywords with
SD-identical defaults (verified pixel-for-pixel against the committed
sheets); smooth swaps NEAREST for LANCZOS/BICUBIC. tools/hd_assets.py
derives the 4x WebP tier from assets/raw sources: flood-key, crop,
margin/fit/center without palette quantization, then renders 4x sheets
through a temp PNG bridge.
