robot_assets: put the CAD toolchain behind a cad extra

[T-K-233]

Why

robot-assets ships two unrelated halves in one distribution:

• robot_assets/loader.py — a stdlib-only GitHub ZIP fetch/cache helper (load("robots/lite/urdf/lite.urdf")), which is what runtime consumers want
• robot_assets/workflow/* — the CAD → URDF/MJCF/xacro generator

Both were covered by one hard dependency list, so installing this package for the loader also pulled mujoco, onshape-to-robot and pymeshlab — none of which the loader imports. A description package should be asset-generation tooling plus assets; deployment code consuming it shouldn't inherit a simulator.

The ROS side was already clean (package.xml only exec_depends on xacro), so this only ever leaked through the Python wheel — but it leaked to exactly the consumers we care about.

What

• dependencies = []; the three move to [project.optional-dependencies] cad
• test group depends on robot-assets[cad], so CI needs no change — it already runs uv sync --group test
• Guarded the two entry points that need the toolchain: urdf_to_mjcf.py's import mujoco, and the onshape-to-robot CLI lookup in onshape_to_urdf.py (a subprocess, so a missing install would otherwise fail obscurely). Both now name the extra.
• README: uv sync --extra cad for regeneration; noted the dependency-free base install

On the name: extras in the wild are nouns for the capability you're opting into (mujoco itself ships sysid/usd), so cad over a verb like generate. It also matches this repo's own vocabulary — robots/<robot>/cad/ is exactly the directory holding the inputs these three deps consume.

Two notes kept inline in the extra, since neither is obvious from the dep list:

• MuJoCo is the MJCF compiler, not a simulator — urdf_to_mjcf.py uses MjModel.from_xml_path + mj_saveLastXML. There's no standalone binding for it.
• pymeshlab is really onshape-to-robot[pymeshlab] — never imported by us, required because every cad/config.json sets "simplify_stls": true. Kept as an explicit pin to preserve the >=2025.7.post1 floor.

Downstream

prime_description declares a plain robot-assets git dependency, with a comment noting it "transitively provides mujoco / onshape-to-robot / pymeshlab" — which stops being true here. It imports robot_assets.workflow, so it needs robot-assets[cad] @ git+.... That change is prepared and must merge after this one, since the extra has to exist upstream first.

The same change is up for Lite-Specialist-Description (T-K-233/Lite-Specialist-Description#1).

Verification

• Base install is exactly one package with zero transitive deps, and from robot_assets import load works in it
• Both guards fire with their intended messages in that bare env
• uv sync --group test resolves the self-referential extra correctly
• pytest: 1569 passed, 6 skipped; flake8 clean on touched files (the one W503 in urdf_to_mjcf.py predates this branch)

🤖 Generated with Claude Code

https://claude.ai/code/session_01FwK47CB1Cdu2PEYtqt87Js