URDF export for robot simulators (#719) (#942)

A design with components and joints exports as a URDF robot description (file.export to .urdf, the MCP export tool, solidcraft-cli run --out, File > Export URDF...). It writes one binary STL per component in metres, links with mass, centre of mass and inertia from material density, and maps rigid, revolute, slider, cylindrical, pin-slot, planar and ball joints to URDF joints or short chains. Loops, broken joints and loose parts are reported as warnings. Options: mesh_prefix, effort, velocity. Docs in docs/urdf.md.

skills/solidcraft-modeling/SKILL.md

@@ -56,7 +56,7 @@ Work the part out as numbers first. Then build it in a few large `batch` calls,
   - When the server is bridged, `source` defaults to the whole app window and `view` also turns the user's camera, so pass `"source":"model"` and leave out `view` unless the user wants the camera moved.
 
 ## Finish
-`export {path}` writes `.step` for CAD or `.3mf`/`.stl` for printing. `save {path}` writes `.solidcraft`.
+`export {path}` writes `.step` for CAD, `.3mf`/`.stl` for printing, or `.urdf` (an assembly with joints, for robot simulators). `save {path}` writes `.solidcraft`.
 
 ## Red flags