MultiColumnNode
data class MultiColumnNode(val children: List<PdfNode>, val count: Int, val gap: Dp, val spacing: Dp) : PdfNode
Newspaper-style multi-column flow: children are measured at the column width and distributed left-to-right into count equal-width columns, balanced so the columns end up roughly the same height.
Children keep their source order — the first children fill the first column. The whole block participates in page breaking as one unit (columns don't continue across pages); split long content into multiple columns { } blocks when it can exceed a page.