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/step-import.md

@@ -39,3 +39,9 @@ display on the resulting body.
 ## IGES
 
 IGES 5.3 files (`.igs`/`.iges`) open, insert and drop the same way: `crates/kernel/src/iges_in.rs` (written from the published specification) restates the file as STEP and the STEP reader reads it, so the feature holds STEP text. Read: manifold solid B-reps (186) with their shells, faces, loops, edge and vertex lists; trimmed (144) and bounded (143) surfaces, and bare B-spline patches, sewn into a solid by their shared edges (an open body when they do not close); lines, circular arcs, ellipses, composite curves, rational B-spline curves; planes (108, 190), cylinders, cones, spheres, tori, rational B-spline surfaces, surfaces of revolution and tabulated cylinders; transformation matrices; units; colours (314 and the predefined numbers); body names (name properties, else labels). Not read yet: ruled and offset surfaces, curves on surfaces given only in parameter space. Export (`file.export` to `.igs`) writes each body as a manifold solid B-rep on the same surfaces a STEP export uses, with its name and colour.
+
+## FreeCAD and Rhino
+
+FreeCAD `.FCStd` and Rhino `.3dm` files open, insert and drop the same way, in the desktop app and the CLI. They are read with [cadmpeg](https://github.com/cadmpeg/cadmpeg) (Apache-2.0, `crates/io/src/native_cad.rs`): cadmpeg detects the format from the bytes, decodes it into its neutral model under resource caps, checks that model and writes it as STEP AP214, which the STEP reader then reads. The feature holds that STEP text, so the design reopens without the original file. The file's solids come in; its feature history, sketches and parameters do not. Losses cadmpeg reports become warnings on the feature, and a file without an importable solid is refused with a message. cadmpeg runs inside the panic guard. How much of a file is read follows cadmpeg's support for that format ([format support](https://github.com/cadmpeg/cadmpeg/blob/main/docs/format-support.md)).
+
+Only codecs for open formats are compiled in: the workspace turns off cadmpeg's default features and enables `fcstd`, `rhino` and `step` (the writer). cadmpeg's codecs for commercial CAD formats are not built, and neither is its `sat` codec, which decodes ShapeManager streams with ACIS as one dialect, so ACIS `.sat`/`.sab` files are not read yet. `crates/io/src/native_cad.rs` is the one place cadmpeg is called; a cadmpeg-based STEP reader could be added there later behind a setting, to compare with the STEP reader above without changing it. The browser app does not read these formats yet: cadmpeg compiles the zstd C library, which the wasm build leaves out.