v0.21.0 — three Etch generators an agent can drive

Three new Etch tools, taking the count to 223.

• etch_make_living_hinge — rows of slits that let a flat sheet bend. The layout rules are why this is a tool rather than a few hundred etch_add_element calls: alternate rows must be offset half a period or the panel does not bend at all, and a slit reaching the edge of its region is a split the panel tears along on the first fold. Returns the tightest right-angle fold it will take.
• etch_make_perforation — a field of holes for a grille, a vent or a diffuser. Returns the narrowest web, the material left between two neighbouring holes, which decides whether the panel survives being cut and is invisible in any of the settings that produced it.
• etch_make_ornament — guilloche, a maze, an animal print or vine scrollwork, behind one kind. Settings are validated against that generator's own field list, so an option it does not have comes back naming the ones it does rather than being silently dropped.

All three accept their numbers as numeric strings as well as numbers, and each returns its drawing as one compound path element rather than hundreds.