ConstThe tile geometry lives in the SDK (IFill.toCSS bakes it into a repeating SVG data-URI), so
this asks the model for a pattern fill's CSS rather than keeping a second, drifting copy of the
twenty-odd Excel tiles. That makes it generic: any BuiltInSheetPattern with any two colors,
whether it is a menu swatch, a preview, or a legend.
One pattern tile, drawn exactly as the grid draws it.