The lower BlueScale clamp from #21343 guarded foundry fonts via
`blueScale < DEFAULT_BLUE_SCALE`, but that lets a near-default value
(e.g. 0.037) with small zones get raised to `0.5 / maxZoneHeight`. On
macOS' Core Text rasterizer this collapses the overshooting glyphs, so
most text disappears (not reproducible on Linux/Windows).
