galleryRowSpan function
How many coarse ROWS a tile of the given aspect spans in the CSS-grid
base. Buckets mirror the previous masonry (square ~2, portrait/wide ~3,
tall ~4) against the minColumnWidth/2 row unit.
Implementation
int galleryRowSpan(double aspect) {
final double a = _clampAspect(aspect);
if (a <= 0.62) return 4; // tall
if (a <= 0.84) return 3; // portrait
if (a >= 1.28) return 3; // wide / panorama (also spans columns)
return 2; // square
}