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]>

crates/doc/src/format.rs

@@ -229,6 +229,10 @@
 //!   transformation: a link without `coupling` is a plain motion link, as it always was; 0.13.0
 //!   (which reads up to 22) asks for an update instead of keeping a gear's ratio when its radius
 //!   changes.
+//!   Also under 23, an Import's `file` can name a FreeCAD `.FCStd` or Rhino `.3dm` file (#1435,
+//!   read through cadmpeg); its `step` is still the STEP text the file was restated as, so the
+//!   feature's fields are unchanged and version 22 designs need no transformation; 0.13.x asks
+//!   for an update before it meets these imports.
 
 use serde_json::Value;