Import FreeCAD and Rhino files through cadmpeg (#1435)

FreeCAD .FCStd and Rhino .3dm files open, insert and drop like STEP and IGES:
cadmpeg (Apache-2.0, pinned =0.6.1, native builds only) detects the format,
decodes it under its service resource caps, validates the model and writes it
as STEP AP214, which becomes an Import feature in a new component named after
the file, with the up and names options.

Only the fcstd, rhino and step codecs are compiled in (no default features).
ACIS is left out: cadmpeg's sat codec is a ShapeManager stream decoder.
native_cad.rs is the single importer seam. Self-made FreeCAD and Rhino box
fixtures, hostile-input, ZIP-bomb and cap tests, engine tests for the
component path; the browser build reports these formats as not supported.
A note under design format 23 says an Import's file can name these formats.

Co-authored-by: Jacob Oursland <[email protected]>
Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/book/src/guide/files.md

@@ -6,6 +6,7 @@
 | `.json` (self-contained plain JSON design) | yes | yes |
 | STEP (AP203, AP214, AP242) | solids, assemblies, units, names, colours | AP242 with names, colours and the component tree |
 | IGES 5.3 | manifold solids, or trimmed surfaces sewn into solids | manifold solids with names and colours |
+| FreeCAD `.FCStd`, Rhino `.3dm` | solids, through cadmpeg (desktop app only) | |
 | 3MF, STL, OBJ | mesh bodies | yes |
 | glTF 2.0 (`.glb`, `.gltf` with its `.bin` files) | mesh bodies in the node tree's components, materials as appearances | |
 | 3MF with face tags | as 3MF | each triangle tagged with its B-rep face, exact coordinates |
@@ -13,7 +14,7 @@
 | DXF, SVG | into a sketch | sketches and flat patterns as DXF |
 | PNG, JPEG | canvases and decals | |
 
-Opening a STEP or IGES file creates a design with the file's bodies as a base feature, and
+Opening a STEP, IGES, FreeCAD or Rhino file creates a design with the file's bodies as a base feature, and
 inserting one adds them to the current design. Later features build on them. Mesh bodies render,
 measure, move and export, but solid features need B-rep bodies.