robot features: URDF-aware [pose] in loon, and a swept-clearance command

[ecto]

Two things rose-pro built OUTSIDE vcad that any robot project will need:

1. Posed assemblies. rose's pose.py wraps each module root in translate/rotate chains per kinematic chain — it works and cache-hits, but the joint tree already exists in the URDF vcad can import. A [pose] form (or vcad render --pose '{joint: deg}' on an imported URDF) makes any robot poseable natively.
2. Swept clearance as a product. rose's sweep.py / skin_sweep.py — rotate child-side geometry through each joint range against the parent side, report min gap — is the actual safety gate of the whole project and it's 700 lines of project Python. vcad sweep robot.urdf --ranges ... reporting min clearance per joint (and the angle at which it occurs) would be a headline feature; the kernel already has signed distance everywhere it needs it.

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