Updates:
Transparent-pixel colour — exported PNGs keep colour under erased (alpha 0) areas; invisible, but still encoded. Flattening it: 522 KB → 4.3 KB on WebP (−99%), −75% end to end.
Redundant channels — greyscale stored as RGB pays for two empty channels: −26% on PNG.
Chroma by measurement — SSIM is luma-only, so 4:2:0 moved it just 0.0004 while doing real colour damage. Now simulates the chroma round-trip and counts shifted pixels: photos 0.00%, coloured text 1.5–2%. −8% to −10% on images the old rule forced to 4:4:4, for ~1 ms.
Denoising rejected — looked like −43% until measured as size at matched SSIM; it lost at every target. Not shipped, documented as such.