Combining animation.Scale with animation.Translate to center the zoom
(two commits back) visually fixed the top-left crop bias, but introduced
a worse problem: pixlet periodically re-rasterizes the whole image with a
visible "pop" roughly every 0.7-1s, confirmed with both WebP and GIF
output and independent of zoom amplitude -- a property of pixlet's
animation engine when Scale and Translate are combined, not something
fixable from this app's math. Scale alone, with no Translate, has none of
this popping (confirmed via isolated testing).
Reverted _art_widget to Scale-only (as it was before centering), and
restored ZOOM_LEVELS["strong"] to 1.7 (the strong-zoom black border only
applied to the Translate-based centering, not plain Scale). This brings
back the original top-left crop bias as a known, accepted cosmetic
trade-off -- better than a periodic visible pop. Updated the README and
dev/calibration_test.star to match and documented the full history for
anyone who wants to revisit centering later.