crates/engine/src/print/layout.rs is the pure page model the Print module
builds on: ten named papers plus custom, orientation, margins and the
printable area; Single Image, Contact Sheet (grid with spacing and square
cells), Picture Package and Custom Package styles; package cells with a
stacking order (send to back / backward / forward / bring to front); a
deterministic MaxRects packer behind Auto Layout (largest first, earlier
pages filled before a new one opens, a cell turned a quarter only when it
fits nowhere as given); snap-to-cells and snap-to-grid helpers; ruler units
and guide options.
All lengths are inches in page space, after orientation. Every number from
outside is checked (NaN, infinity, negative, oversized, cell and page caps)
and Layout::validate guards saved layouts.
No command or UI reaches it yet, so the parity rows keep their status.