Render a part to images

Render a part to PNG images so you can look at it. No display or GL is needed (CONTRACTS.md section 15).

A look before the first run and after each run finds a wrong region or load fast.

Input: stl_paths (STL files; roles: one role for all or one per file, from part, design, fixed, load, keep-out; default part; an item may be a comma list), or run_dir (renders result/final.stl and the region STLs that run.json names; missing region files are listed in the facts), or yaml_path (a problem YAML, before a run: the region STLs and the interface shapes. Fixed, load and union shapes are solid. The design space, keep-out regions and cut shapes are ghosts. A face is its cut zone, clipped to the grid box. Look at it before a run: it shows what a cut takes). With none of them, the last run is used. A YAML render also draws the force arrows (one for each force of each load case, a color for each case, along the force, ending where it acts): you can show it to the user to check the directions before the run. The answer is a JSON text and, when inline is true, the montage as one image. With inline false the answer is the text only; the PNG files are on disk and a client that can open files reads them. A 4-view render of an 85,000-triangle part takes about 2 to 4 s; the first call with an empty numba cache adds under 1 s. Later renders overwrite PNGs with the same names; out_dir picks another folder. out_dir: default /renders, or renders beside the first STL. max_triangles: refuse meshes with more triangles than this (default: no limit; the facts give a memory estimate, about 450 MB plus 450 MB per million triangles). Reply keys: resource topopt://replies/render_part (the meaning of each key). The facts of each mesh have watertight and winding_consistent. loads: false draws no force arrow in a YAML render (default: arrows). A moment and a pressure get no arrow, and the facts say so. The arrow length is a share of the part size: it does not show the force size. views: names from iso, front, back, right, left, top, bottom (default iso, front, right, top); NAME:dx,dy,dz gives your own direction from the part to the camera, z up (name: letters, digits, _, -). section: "x=30" (or "x>=30") keeps x >= 30; "x<=30" keeps x <= 30; the axis is x, y or z, in mm. regions: how the non-part meshes are drawn. ghost (default with a part): design and keep-out as faint ghosts, fixed and load as strong ghosts, so the part stays visible. solid (default without a part). off: the part only. size: one tile in pixels, WxH (default 640x480). The montage is at most 1568 px on the long edge. All tiles have one scale; a scale bar shows it. In views, one item may be "fl:-0.5,-1,0.25". A text like "iso,fl:-0.5,-1,0.25,top" in one item is read as three views. Cut faces are flat orange. With a section and no views, the views are: cut (straight at the cut face from the removed side), iso-cut, and two side views of the kept part. The facts give cut_face_visible_px for each view; they say if a mesh could not be cut (for example, it is not a closed solid), and the render is made anyway.

Colors: part grey-blue, design space light blue, fixed dark blue, load red, keep-out brown, cut face orange; for a YAML also union shape green and cut shape (a ghost) pink-red. Back faces are magenta: they show a flipped or open surface. The same input gives the same PNG bytes.