Returns per-set geometry for a true Venn diagram of set_names.len() sets,
where every one of the 2^n - 1 regions is present. Used by the R venn()
path for shapes whose layout is supplied by eunoia rather than by eulerr's
precomputed ellipse table. Currently only "rotated_rectangle" is wired
here (it supports n in 1..=4); the rotation is what lets four rectangles
open all 15 regions, which axis-aligned rectangles cannot.
venn_layout(set_names, shape)