composeAtlas blits placed tile pixels into bounds-sized RGBA + L8 PNGs,
synthesizing L8=0 planes for unpaired diffuse tiles. buildAtlasJson
emits the metadata with deterministic ID-sorted tile arrays and applies
the optional blocks_sight regex.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
