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]>

README.md

@@ -136,7 +136,9 @@ rebuild with no errors, replayed here at 2× speed.
   STEP import (AP203/AP214/AP242 solids, assemblies, units, names, colours) and IGES import
   (manifold solid B-reps, or trimmed and bounded surfaces sewn into solids) as a base feature
   that later features build on — open a `.step`/`.stp`/`.igs`/`.iges` file, insert one into a
-  design, or drop it on the window ([docs/step-import.md](docs/step-import.md)).
+  design, or drop it on the window ([docs/step-import.md](docs/step-import.md)); FreeCAD
+  `.FCStd` and Rhino `.3dm` solids import the same way through
+  [cadmpeg](https://github.com/cadmpeg/cadmpeg) in the desktop app and CLI.
 - **Render:** opaque surfaces can use metal fraction, roughness and dielectric IOR in the
   integrated product viewport and CPU preview PNGs. Existing simple colors remain supported.
   Fixed direct lighting is a preview; HDRI, refraction and texture maps are not implemented.