Function set_icon 

pub fn set_icon(
    doc: &Document,
    block: BlockId,
    asset: &Asset,
    builder: &mut CommitBuilder,
)

Inventory row “Set Block Icon”: attach — or replace — a block’s foreground artwork. The icon is one atomic value, so the picture and the box it sits in are written whole; the default box is a square ICON_SIDE of the block’s shorter side, centered.