[czj]

Cards, catalogues and blocks of addresses side by side needed a row per line and widths computed by hand, and the last, shorter line came out with wider cells than the others. Part of #71.

grid(columns:) puts its cells columns to a line, all as wide, the last line's included. It is a box like the others, on the document or in a stack or a row.

• Lines move whole: a line is as high as its tallest cell and goes whole to the next page; pages break between lines, never inside one.
• Cards line up: a cell with a background or a border is drawn as high as its line, like a CSS grid's.
• Two gaps: gap is one number, or [between lines, between columns].
• Kept with the next: a cell with keep_with_next keeps its line with the next one, a heading in a cell for instance.
• Wrong input raises: columns that is not a positive integer, a cell's width or grow, and gaps wider than the grid. Padding, background, border and keep_together work as on a stack.

Proof

test/native_grid_test.rb covers column positions, both gaps, line height, stretched decorations, page breaks between lines, keep_with_next, a decorated grid, a grid in a row, and the errors. The contract lists #grid on the document and on containers. A 40-card catalogue runs over two pages with whole lines and cards of equal height, and qpdf --check passes. rake test: 1333 runs, 0 failures; RuboCop is clean.