Fix cell_to_svg viewport for cells with references
Compute bounding box from flattened elements instead of the cell
directly, so cells containing only references get a correct viewport.
Also add SVG export to the sample example.
Enable grid by default and increase line visibility
Grid was defaulting to off (bool default = false). Switch to explicit
Default impl with show_grid: true. Also brighten grid lines from
rgb(50,50,50) to rgb(60,60,60) and origin axes to rgb(100,100,100)
for better contrast against the rgb(30,30,30) background.